the Tunicata and Brachiopoda, to the rank of a sub-kingdom. This, as its author admits, is a merely provisional step; and it is quite possible that, as the result of yet further investigation, they may merge once more in the primary division from which they have been separated. It is but lately that the general plan of the nervous system has been determined, and we are still far from a thorough comprehension of its details.*

It is not my purpose, however, to discuss the systematic position of the group, but simply to notice a few of the more interesting points in its history, and especially to place before the readers of the POPULAR SCIENCE REVIEW Some of the most important results of recent research.

And, as a preliminary, it may be useful to give a slight sketch of the form of structure which is characteristic of the Polyzoa. In the first place, they are universally composite

h

Fig. 1.

b

d

a

animals. The primary zooid, the immediate product of the egg, gives origin at once by budding to a second, which remains in organic union with it; and this multiplication by continuous gemmation proceeds on a definite plan, but to an indefinite degree, during the lifetime of the animal. In this way colonies of greater or less extent are formed, in which a multitude of zooids (polypides) combine individual independence with subservience to a common life.

Each polypide is enclosed in a cell (fig. 1, a), which is horny or membranous or calcareous, as the case may be, and is furnished with an aperture, usually placed at one extremity, through which the polypide protrudes at pleasure the anterior portion of its body, bearing a wreath of ciliated tentacles (fig. 1, b), in the centre of which is placed the mouth. This aperture is, in some cases, closed by a movable lip, and in others by a much more elaborate operculum. The incessant and vigorous play of the cilia creates a perfect whirlpool within the tentacular crown, by which alimentary particles are borne through the central mouth into a wide

i

It is worthy of remark, in this connection, that the important discovery of a "colonial nervous-system," uniting and bringing into relation the many zooids composing a polyzoan community, which we owe to Fritz Müller, and of which we shall have more to say hereafter, is barely referred to by Huxley in his recent work, and is dismissed with the remark that it requires confirmation.

pharyngeal chamber (fig. 1, c). From this, by powerful contractions, they are shot down the oesophageal tube (fig. 1, d), either into a gizzard (fig. 1, e), where they are triturated, or, in the absence of this organ, directly into the stomach itself (fig. 1, f), a bag or sac, which is usually of a rich reddishbrown colour, and, with the rest of the viscera, is suspended in the cavity of the cell. Here they are rapidly digested. The unuseable portions are sifted out and borne by the action of cilia towards the upper end of the stomach, which has the appearance of being doubled upon itself; and, after rotating here for a time, they pass through a valvular orifice (fig. 1, g) into the intestine (fig. 1, h), a long and slender tube which is continued upwards to an outlet at the base of the tentacles. The space that intervenes between the body of the polypide and the walls of the containing cell (the perivisceral cavity) is filled with fluid, into which the products of digestion find their way, it is supposed, through the walls of the stomach, and are thus made available for the nutrition of the common life. In this cavity the reproductive organs are developed at certain seasons of the year.

The polypides are lively and rapid in their movements; and a most interesting sight it is to see them issuing from their little dwellings, and suddenly expanding their exquisite bells of tentacles, clothed with a thousand vibrating cilia, and then on the slightest alarm darting back again with the speed of light. A well-developed muscular system supplies the means of all this activity.

A simple nervous system has long been recognised in each polypide, consisting of a single ganglion placed between the œsophagus and the rectum, from which filaments pass off in various directions, and especially, it would seem, towards the tentacular region.* Such is the general plan of structure.

It is impossible to frame any general description of the aspect and habit of the Polyzoa. These are as various as the modes of gemmation. They grow in plant-like tufts, stem and branches composed of series of cells linked together; they spread like lace-work, or as fairy chains, over other bodies; they rise into stony coral-like masses; they cover the fronds of the seaweed with fleshy crusts or silvery network; they form colonies of exquisite little frosted tubes. In many of the stony kinds the cells are richly sculptured, the single genus Lepralia exhibiting an immense variety of microscopic ornament.

Nitsche, in an able and interesting paper on the fresh-water Polyzoa, has described and figured an ample and most effective service of nerves in the lophophore of the well-known Alcyonella.

Such is the class in general character. I proceed to notice the points of special interest to which I referred at first.

The increase of the polyzoan community by external budding is one of the most familiar facts in its history; but it is only lately that we have learnt that a provision exists for the renewal of the polypides, and that as one generation of them disappears its place is soon filled by another, which occupies the same cells and perpetuates the life of the colony. In the Hydroid Zoophyte the life of the individual polypites is evanescent ; they soon perish, and are followed by others which bud from the common flesh. Amongst the Polyzoa also it appears that a succession of zooids may occupy the same colony, but the mode in which the second generation is supplied differs widely from that which obtains amongst the Hydrozoa. Students of the former class have long been familiar with certain dark-reddish bodies, of somewhat spherical form, which are commonly present in the cells, and remain in them after the death and disappearance of the polypides. Not unfrequently whole colonies of Bowerbankia for instance are found, in which almost every cell is tenantless, but exhibits a single dark spot.

These bodies have been commonly regarded as ova, which are not liberated till after the death of the polypide; but they have, as we shall see, a totally different significance, the discovery of which has opened to us one of the most deeply interesting chapters in the polyzoan life-history. Let us first trace them to their origin, and then enquire into their function. At certain seasons a very marked change is seen to be taking place towards the base of the body of the polypide (fig. 1, 2). This change consists in the gradual separation of the lowest portion of the stomach from the rest of that organ. After a time a somewhat spherical dark-reddish mass is found to be, as it were, suspended below the stomach, with which it is still connected by a narrow channel. In this semi-detached portion the characteristic contraction of the walls, by which the food is driven upwards within the digestive cavity, may yet be noticed. At this stage there is no absolute separation, but merely a contraction of the stomach, by which its lowest portion is formed into a distinct chamber. At length, but whether before or after the death of the polypide I am unable to say, this inferior section of the digestive sac is completely cut off from the rest and lies within the cell as a separate structure. In this condition it is found to consist of a granular mass enveloped in a membrane, which is thickly covered

on

The interesting facts which I am about to detail were first fully brought to light by the Swedish naturalist Smitt, in a very able paper 66 the Development of the Marine Bryozoa." His observations have been confirmed by Nitsche; and I have myself verified them again and again.

with pigment spots; and it is now "the dark body" so long known to the student of the Polyzoa, and, as it proves, so falsely interpreted. In the species which I have most carefully investigated it remains attached to the cord that connects the polypide with the base of its cell; it occupies indeed much the same position as it did when an integral part of the digestive system. The original tenant of the cell having run its course and disappeared, the bud from its own substance which it has left behind it enters upon a course of development resulting in the formation of a new polypide. It first increases in size at the expense of a mass of fatty globules, by which it is more or less surrounded; then a small bud makes its appearance on its upper surface, which is gradually moulded into the rudiments of an alimentary canal and wreath of tentacles. Development proceeds step by step, until at last a fully formed zooid has been evolved out of the remnant of its predecessor, and the cell is once more in possession of a tenant.

How often this process may be repeated we have at present no means of judging; but the curious provision which I have just described for supplying the colony with at least a second generation of polypides seems to be universal amongst the marine Polyzoa.'

I have already referred briefly to the simple nervous system which has long been recognised in the individual polypides. Recent investigations have shown the existence of a "colonial nervous system" besides, by which the many zooids in a community are united and brought into relation. The original observations of Fritz Müller have received confirmation from Smitt; and I have also been able to trace the common nervous system in a considerable number of species. In the Polyzoa, such as Bowerbankia or Valkeria (fig. 1), in which the cells are set singly or in clusters on a distinct stem which is horny and more or less transparent, it is readily detected. This is not the place for details, but it may be stated generally that a nervous trunk pervades the stem and branches, connecting a series of ganglia which are situated at the joints, and communicating, by means of nervous filaments that sometimes originate in a complicated plexus, with other ganglia at the base of the individual cells. We have seen this structure beautifully displayed in a branch of Valkeria pustulosa preserved in fluid, within which a rich nervous plexus could be distinctly observed, giving origin to a multitude of most delicate threads that passed from it to the numerous cells composing the neighbouring cluster. In the calcareous species this colonial nervous system is more

• The whole process may be regarded as a mode of gemmation. Smitt names the detached bud the "groddkapsel."

difficult of detection; but it has already been recognised in several of them, and is no doubt present in all. Without attempting to describe its plan in detail, I may mention that I have traced the main nerve-trunk in Bicellaria ciliata, and have observed that a branch passes from it to the base of the avicularium, which is furnished, as we know, with a special apparatus of muscles. The interest and importance of these discoveries will be at once recognised." *

We turn now to the reproductive history of the Polyzoa, which offers a few points of special interest. It has long been well known that at certain times both eggs and spermatozoa are developed in each cell, the former from its membranous lining and usually the upper portion of it, the latter towards the bottom of the cell. At a certain stage of their growth the ova escape from the delicate envelope that has hitherto confined them, and lie free in the perivisceral cavity, where they are fertilised and enter upon the course of development that results in their conversion into active ciliated embryos. Smitt, however, has noticed another mode of reproduction, which he regards as asexual. He describes eggs as occurring within the cell (of Lepralia Peachii), and lying in a loose mass of fatty globules, by which they are nourished. In proportion as the ovum increases in size this mass diminishes around it, until at last it lies free in the cavity. At this stage it is furnished with a membrane and is of a dark red colour. It afterwards undergoes segmentation, and passes into the condition of a ciliated embryo; but no spermatozoa have ever been detected in the cells in which such ova are developed. The observation is a very interesting one, and to a certain extent I can confirm it. But it is to be remarked that the spermatozoa are difficult to detect, and we require the evidence of repeated observation to assure us of their non-existence. Smitt has recorded the occurrence of this asexual mode of propagation in several species, and names it "the formation of ova by internal budding."

The structure of the curious embryo, which is the immediate product of the egg, has been thoroughly investigated by Nitsche and illustrated by admirable figures. It exhibits some very interesting points. The shape of the embryo varies in different species, and is often difficult to describe, but the plan of structure seems to be pretty uniform throughout the class. whole surface of the body is covered thickly with cilia, which exhibit some diversities of size and form, and by means of which it swims rapidly through the water. Its movements are

The

* Reichert has challenged Müller's conclusions; but his criticism seems to be based on the examination of a single species, and is not sustained by the results of further investigation.

† Beiträge zur Kenntniss der Bryozoen. Von Dr. Heinrich Nitsche. 1869.