In some Cystoidea there were no arms, properly speaking, but only small pinnulæ. In a second section two arms were present, but these were bent backwards, and were immovably soldered down to the body. In one single species (Comarocystites punctatus, Billings), the development has gone further, the arms being free, and provided with lateral pinnulæ, as in the true Crinoids.

Many Cystideans are likewise provided with a system of pores, or fissures, penetrating the plates of the body, and usually arranged in definite groups. These groups are termed "pectinated rhombs," but their exact function is doubtful. By Mr Billings, however, they are believed, and apparently with good reason, to have admitted water to the body-cavity, and to have thereby subserved a respiratory function.

ORDER BLASTOIDEA.-Body enclosed in an armour of closelyfitting calcareous plates, attached to some foreign body by a slender stem. From the summit of the calyx radiate five transversely-striated and longitudinally-grooved areas, which carry a row of jointed pinnula on each side.

The members of this order, like those of the preceding, are all extinct, and are entirely confined to the Paleozoic period.

Fig. 96.-Morphology of Blastoidea. a Pentremites pyriformis, viewed side-ways, showing a portion of the column; b Summit of the calyx of Pentremites cervinus, showing the pseud-ambulacral areas and the apical apertures; c Side-view of Granatocrinus melonoides; d Summit of Granatocrinus neglectus. (Figs. a and b are of the natural size; c and d are slightly enlarged.) After Hall, and Meek and Worthen.

The body was fixed to the bottom of the sea by means of a short, jointed pedicle; it was globular or oval in shape, and composed of solid polygonal calcareous plates, firmly united together, and arranged in five inter-ambulacral and as many ambulacral areas. (These ambulacral areas are termed by M'Coy "pseud-ambulacra," upon the belief that they were not pierced for tube-feet, but that they carried a double row of

little jointed tentacles or arms.) The pseud-ambulacra are petaloid in shape, having a deep furrow down the centre, and striated transversely They converge to the summit of the calyx, and each appears to have carried a row of small jointed "pinnula" upon each side. The five pseud-ambulacra, radiating from the summit of the calyx, give the upper surface of the body somewhat the appearance of a flowerbud; hence the name applied to the order (Gr. blastos, a bud; eidos, form). Upon the whole, it would seem most probable that the pseud-ambulacra of the Pentremiles represent the arms of the Crinoids, anchylosed with the calyx, and that the longitudinal furrows of the pseud-ambulacra represent the " brachial grooves" of the Crinoids.

66

At the summit of the calyx are six apertures, of which one is the mouth, four are ovarian, and the sixth is probably partly ovarian and partly anal.

The Blastoidea are known more familiarly under the name of Pentremites, and they occur most commonly in the Carboniferous rocks.

CHAPTER XXII.

HOLOTHUROIDEA.

ORDER HOLOTHUROIDEA.-- Vermiform or sluglike Echinoderms, with a leathery skin, in which calcareous granules and spicules are generally developed. Mouth surrounded by a circlet of tentacles. Sexes mostly distinct. Larva vermiform, without a skeleton.

The members of this order are commonly known by the

name of " sea-cucumbers," "trepangs," or "bèches-de-mer,"

and are the most highly-organised of all the Echinodermata. The body is elongated and vermiform, or rarely slug-shaped, and is not provided with a distinct test, but is enclosed in a coriaceous skin, almost universally containing calcareous deposits in the form of scattered granules or spicules, or even imbricated scales. The ambulacral tube-feet, when present, are typically disposed in five rows, which divide the body into an equal number of longitudinal segments or lobes. The mouth is surrounded by a circlet of feathery tentacles, containing prolongations from the central ring of the water-vascular system; and an anus is situated at the opposite extremity of

the body. There is a long, convoluted intestine. A special respiratory, or water-vascular, system is often developed, in the form of a system of arborescent tubes, which admit water from the exterior. The larva is vermiform, and has no skeleton. At a certain period of their existence, the young Holothurians are barrel-shaped, with transverse rings of cilia (fig. 97, c).

They rotate rapidly on their long axis, and have at this stage been described as a distinct genus under the name of Auricularia.

In the typical Holothurians, locomotion is chiefly effected by means of rows of ambulacral tube-feet, or by alternate extension and contraction of the worm-like body; but in the Synaptida, in which there are no ambulacra, and only the central circular canal of the ambulacral system is present, the animal moves by means of variously shaped spicula, which are scattered in the integument. When developed, the ambulacral system consists of a "circular canal," surrounding the mouth, bearing one or more "Polian vesicles," and giving off branches to the tentacula; and of five "radiating canals" which run down the interspaces between the great longitudinal muscles. These radiating canals give off the tube-feet and their secondary vesicles, just as in the Echinus. In the typical forms there are five rows of tube-feet, but these organs may be scattered over the whole body, or may be restricted to the ventral sur

face. There is also a "sand-canal" which arises from the circular canal, and is terminated by a madreporiform tubercle; but this, instead of opening on the exterior, hangs down freely in the perivisceral cavity. The fluid, therefore, with which the ambulacral system is filled, is derived from the perivisceral cavity, and not from the exterior, as is usually the case.

The mouth is toothless, is situated anteriorly, and is surrounded by a beautiful fringe of branched, retractile tentacles (fig. 98), which arise from a ring of calcareous plates, and into

which are sent prolongations from the circum-oral ring of the ambulacral system. These tentacles, ten to twenty in number, are really modified tube-feet, and probably serve in part as respiratory organs. The mouth opens into a pharynx, which conducts to a stomach. The intestine is long and convoluted, and usually opens into a terminal dilatation, termed the "cloaca," which serves both as an anus and as an aperture for the admission of sea-water to the respiratory tubes. From the cloaca arise, in many forms, two branched and arborescent tubes, the terminations of which are cæcal. These run up towards the anterior extremity of the body, and together constitute the so-called "respiratory tree." They are highly contractile, and they perform the function of respiratory organs, sea-water being admitted to them from the cloaca. The vascular system consists of two main vessels-one dorsal, and the other ventral-connected with a circum-oesophageal ring. Development is in a few instances direct; but in most cases there is a metamorphosis, the larva being vermiform, and devoid of a skeleton. The nervous system consists of a cord, surrounding the gullet, and giving off five branches, which run alongside of the radiating ambulacral canals. The sexes are generally, but not universally, distinct. The generative organs are in the form of long, ramified, cæcal tubes, which open externally by a common aperture, situated near the mouth. There is thus no trace of that radial symmetry which is observed in the arrange

ment of the reproductive organs in the other orders of the Echinodermata.

The skin in the Holothurians is highly contractile, and the body is provided with powerful longitudinal and circular muscles, in compensation for the absence of any rigid integumentary skeleton. Many of the Sea-cucumbers, in fact, are endowed with such high contractility that they can eject their internal organs entirely, if injured or alarmed.

In the family of the Synaptide there is no respiratory tree, and the ambulacral tube-feet are wanting; whilst the skin is furnished with caicareous spicules of various shapes. The Synapta themselves burrow in the mud or sand, and have the skin furnished with innumerable anchor-shaped spicules attached to special "anchor-plates" in the integument. They often form a kind of protective case or tube of sand-grains; and they obtain their food by swallowing the mud, from which they extract any disseminated nutrient particles. In Chirodota the skin is provided with microscopic calcareous wheels, in the place of anchors. In the Oncinolabida, the skin has barbed spicules, and there is no respiratory tree; but these forms differ from the Synaptide proper in possessing tube-feet. The order Holothuroidea is divided into the two following sub-orders :

Sub-order 1. Apneumona.

No respiratory tree.

The ambulacral tube-feet wanting (Synaptida),

or present (Oncinolabida).

Sub-order 2. Pneumonophora.

A respiratory tree. (Ex. Holothuria, Thyone, Molpadia, Psolus, Cucumaria, &c.)

CHAPTER XXIII.

DISTRIBUTION OF ECHINODERMATA IN
SPACE AND TIME.

DISTRIBUTION OF ECHINODERMATA IN SPACE.-The Crinoidea are represented by very few forms in recent seas, and these have a very local distribution. The Comatulæ are the commonest, and species have been found in most seas. The species of Pentacrinus and Holopus are exclusively confined, as far as is known, to the Caribbean Sea. Rhizocrinus Lofotensis has been dredged on the coast of Norway, and in the North Atlantic, and a form believed to be the same has been found