often evertible; and the intestine may be either straight or branched, but always terminates cæcally behind, and is never

3

1. Planaria torva (Müller); m Mouth; g Nerve-ganglion; e Eyes; ov Ovary; t Testis; gn Genital opening. 2. Planaria lactea, showing the branched (dendrocol) intestine. 3. Microscopic larva of Alaurina, a marine Turbellarian. 4. Pilidium, the "pseudembryo" of a Nemertid : a The alimentary canal; 6 Rudiment of the Nemertid.

provided with an anal aperture. The "water-vascular system communicates with the exterior by two or more contractile apertures. The nervous system consists of two ganglia, situated in front of the mouth, united by a commissure, and giving off filaments in various directions. Pigment-spots, or rudimentary eyes, from two to sixteen in number, are often present, and are always placed in the præ-oral region of the body. The male and female organs are united in the same individual, and the process of reproduction may be either sexual, by means of true ova, or non-sexual, by internal gemmation or transverse fission.

The Planarians have been divided into two sections, as follows:

Section A. RHABDOCELA.-Intestine straight, not branched; body elongated, rounded, or oval.

Section B. DENDROCELA.-Intestine branched or arborescent; body flat and broad.

SUB-ORDER II. NEMERTIDA.-The Nemertida, or "Ribbonworms," agree in most essential respects with the Planarida. They are distinguished, however, by their elongated, vermiform shape, by the presence of a distinct anus, by the possession of a distinct perivisceral cavity, by the absence of an external aperture to the water-vascular system of the adult, and by the fact that the sexes, with one or two exceptions, are distinct. The Nemertida further differ from the other Platyelmia in possessing a pseudohæmal system in addition to, and

P

distinct from, the water-vascular system. The external surface of the body is richly ciliated, and is underlaid by a thick glandular cutis, beneath which are well-developed sub-cutane

Fig. 104-Morphology of Nemertida. Prorhyn chus Auviatilis; 0 Mouth; Ciliated grooves (sense-organs?); s Spine, attached behind to (p) the sac of the proboscis; a Gullet; & Gas

tric glands; i Intestine; ov Ovary. The proboscis in this form is very small. (After Gegenbaur.)

ous muscles. The digestive canal is ciliated internally, and consists of a muscular gullet, a sacculated stomach, and an intestine with a distinct anus. The nervous system consists of two large cephalic ganglia, united by a double commissure, and sending lateral cords backwards. The so-called "circulatory system" is composed of closed contractile vessels, sometimes containing a corpusculated fluid. "Along the median line of the dorsum lies a special muscular sheath, containing a complicated proboscis, and a highly organised corpuscular fluid, both the sheath and the proboscis passing between the commissures of the ganglia in front" (M'Intosh). The evertible and muscular sheath of the proboscis may be as long as the whole body, and the extremity of the latter may or may not be protected by one or more spines (fig. 104). The sexes are mostly in separate individuals, and the generative organs have the form of sacs placed between the muscular walls of the body and the digestive canal, and discharging their contents by lateral pores.

Reproduction takes place by the formation of true ova, by internal gemmation, or by transverse fission. In Nemertes, however, the egg gives rise to a larva, from which the adult is developed in a manner closely analogous to that described as characteristic of the Echinodermata. The larval form of Nemertes was described by Johannes Müller, under the name of Pilidium (fig. 103, 4). It is "a small helmetshaped larva, with a long flagellum attached like a plume to the summit of the helmet, the edges and side-lobes of which are richly ciliated. A simple alimentary canal opens upon the under surface of the body between the lobes. In this condition the larva swims about freely; but, after a while, a mass of formative matter appears on one side of the alimentary canal, and elongating gradually, takes on a worm-like figure. Eventually it grows round the alimentary canal, and appropriating it, detaches itself from the Pilidium as a Nemertid-provided with the characteristic proboscis, and the other organs of that group of Turbellaria" (Huxley). Whilst some Nemertids are thus developed by internal budding from a ciliated provisional larva or "nurse," "others exhibit no such phenomena, but are

directly developed into the adult form, without undergoing any striking metamorphosis.

Whilst the Nemerteans undoubtedly show very close affinities to the Planarians, they are also nearly related to the Annelida proper, both as regards their general form and their internal structure. One of the most remarkable of the links between these two groups is to be found in Balanoglossus, the systematic position of which is still uncertain. This extraordinary worm is found burrowing in the sand in the Mediterranean and along the eastern coasts of North America, and possesses a flattened wormlike body, terminating in front in a protrusible hollow proboscis. The external integument is ciliated, as in the Nemerteans, and there are no setiform organs of locomotion, such as are found in so many Annelides. A watervascular system is present; there is a well-developed alimentary canal; and the process of respiration is carried on by means of a singular branchial sac formed by a chitinous framework with ciliated apertures, somewhat similar to the "branchial sac" of an Ascidian; whilst the sexes are in different individuals. The most remarkable point, however, about Balanoglossus is its mode of development. The larva is a free-swimming, barrelshaped, ciliated body, which was originally discovered by Müller, and described by him under the name of Tornaria, as the young of some Starfish. In spite of its close resemblance to the larvæ of the Echinoderms, A. Agassiz has, however, succeeded in showing that Tornaria is really the young of Balanoglossus, and that it is developed into the adult by a rapid change, not accompanied by any absorption, or casting off, of any portion of the original larva.

The Nemerteans are mostly marine in their distribution, a few forms inhabiting fresh water, and one being found in moist places on land. They are found from the arctic seas to the equator, most of them being littoral in their habits, though some live at considerable depths. Recently, Mr Moseley has described a peculiar group of Nemerteans under the name of Pelagonemertida, which are pelagic in their habits. These oceanic forms have a broad, gelatinous, flattened body, and a ramified digestive tract, and thus make a near approach to the dendrocœlous Planarians. No certain remains of Nemertids are known to occur in the fossil state, though some obscure remains have been referred to this group.

CHAPTER XXVI.

NEMATELMIA.

I. ACANTHOCEPHALA.

2. GORDIACEA. 3. NEMATODA.

DIVISION II. NEMATELMIA. - This section may be considered as comprising those Scolecids in which the body has an elongated and cylindrical shape. Strictly speaking, it should

include the Nemertida, but the division is not founded upon anatomical characters, and is employed here simply for con

Fig. 105.-Morphology of Acanthocephala. Male of Echinorhynchus angustatus, enlarged about twelve times; p Proboscis ; # Neck; & Muscular sheath of the proboscis; g Ganglion; bb" Lemnisci," sacs connected with the wa

venience. Most of the Nematelmia possess an annulated integument; but there is no true segmentation, and there are rarely any locomotive appendages attached to the body. The majority are unisexual, and parasitic during the whole or a part of their existence. Three orders are comprised in this division viz., the Acanthocephala, the Gordiacea, and the Nematoda.

ORDER I. ACANTHOCEPHALA.-Vermiform internal parasites, without mouth or alimentary canal, and having an anterior protrusible proboscis armed with recurved hooks. Sexes distinct.

The Acanthocephala are entirely parasitic, vermiform in shape, and devoid of any mouth or alimentary canal. The front end of the body (fig. 105, ) is developed into a retractile proboscis, which is covered with transverse rows of recurved hooks, and by means of which the parasite attaches itself to the wall of the intestine of its host. The integument (cc) is highly muscular, and the proboscis is contained within a strong muscular sheath, and can be retracted by special muscular bands (mm). At the base of the proboscis is placed a single nervous ganglion, and its hinder extremity is prolonged into 1, the socalled "ligamentum suspensorium," a fibrous band, which supports the generative organs. The sexes are in different individuals. The water-vascular system is in the form of subcutaneous reticulated canals which are connected with two saccular organs or "lemnisci" (bb), placed on each side of the base of the proboscis, but the vessels of this system do not appear to communicate with the exterior.

mentum suspensorium; mm Retractor muscles of the procula seminalis; cc Integument. (After Leuckart.)

boscis; t testes; v Vesi

The order Acanthocephala includes only one genus, namely, Echinorhynchus, the genus Koleops being doubtfully referred here. All the Echinorhynchi inhabit in their adult condition the intestines of fishes, birds, or mammals, and they pass

through a metamorphosis. The eggs are swallowed by crustaceans or insects, and give exit to free vermiform embryos, armed with hooks. These burrow out of the intestine of their host and encyst themselves in its tissues, not becoming finally developed till their bearers may be eaten by some vertebrate animal. Thus, the embryos of Echinorhynchus gigas of the pig inhabit the larvæ of the cockchafer, whilst those of E. angustatus, of Cyprinoid fishes, live in the interior of fresh-water

crustaceans.

ORDER II. GORDIACEA.- Vermiform Scolecida, parasitic in insects during a portion of their existence. An imperfectly-developed alimentary canal or none. Water-vascular system rudimentary or absent. Sexes distinct.

The Gordiacea, or "Hair-worms," are thread-like Scolecids, often singularly like hairs in appearance, which live in the interior of various insects during part of their life. The digestive system is imperfect, an anal aperture being universally wanting. In Mermis, the gullet ends in a blind sac; in Gordius, the digestive tube opens into the body-cavity; and in Sphærularia, the mouth appears to be wanting. The sexes are in different individuals. In Gordius itself, the embryo is free and aquatic, having a retractile snout armed with hooklets, by means of which it, after a time, bores its way into the tissues of some water-insect, in which it encysts itself. sexually-mature worms are found in the interior of Orthoptera or Neuroptera; but they leave their hosts and betake themselves again to an aquatic existence for the purpose of laying their eggs. The adult Mermis is found principally in Lepidoptera; whilst Sphærularia inhabits the body-cavity of Bumblebees.

The

ORDER III. NEMATODA (or Nematoidea).—Cylindrical vermiform Scolecids, sometimes parasitic, sometimes free; integument not ciliated; a well-developed alimentary canal, with a mouth and anus, suspended freely in a body-cavity; sexes distinct, or, rarely, united.

The Nematoda comprise the so-called "Thread-worms" and "Round-worms," and, as their various names imply, possess a rounded and worm-shaped body (fig. 106, a), sometimes of great length. The cuticle is chitinous and porous; and there is generally distinct annulation, though no true segmentation exists. The alimentary system is well developed, the mouth being anterior, and usually furnished with papillæ (fig. 106, c). The gullet opens into a muscular stomach, from which an intestine conducts to a ventrally or terminally placed anus; the whole digestive tube being freely suspended in a body-cavity,