E.CHINODERMATA test], while the side opposite, or towards the central axis of the visceral cavity, is more or less deeply folded longi tudinally " (see fig. 32). These internal canals, as sug gested by Ilofe, may possibly represent the tubes under the dome of the Crinoidea. In Codonaster the ambulacra are confined to the upper portion only of the calyx. Pores and attached tubes are wanting; but there are striated structures between the arms, similar in appearance and probably also iu function to the pectinated FIG. 32. Transverse sec- rhombs of the Cyst idea, their ridges, tion of ambulacrum of as first shown by Rofe (Geol. Mag., Pcntrcmitcs Godoni, -, v - , . x 3. (After Billings. ) 1865, 11. p. 251). being the tops of a , . !, kneci plate ; g, ambulacra] grove ; p, p, pores leading into the canals, c, c. series of folds of a thin test or mem brane, which were perhaps " respira tory sacs, lined with cilia, and constructed of a porous test, through which air from the water could pass by diffusion." The expanded ends of the neighbouring tubes of each two ambulacra form at the summit of the test four double and two single apertures commonly termed " ovarian orifices ; " between the two latter there is usually a third, apparently anal, opening. In Eleutherocriniis there are three paired, and two single pores only. The Blastoiclea, which are (1) represented by Pentremites in Upper Silurian strata, attained their principal development during the Carboni ferous epoch, at the close of which they seem to have become extinct. Order VIII. HOLOTHURTDEA. The Holothuridea, Sea- slugs, Trepangs, or Sea-cucumbers (figs. 33 and 34) have a FIG. 33. Holothuria papillosa. FlG. 34. Holothuria tubulosa. long, cylindroid, sometimes flattened body, -yhich is without shell, and is brown to purplish-red in colour. The peri- some, which is un ciliated, is composed of three layers, a structureless epidermis, a cellular dermis, and an internal elastic layer. The two interior layers contain calcareous spiculae, which vary much according ta the species, and may take the shape of perforated disks, wheels, anchors, and hooks (fig. 35). Rarely the dorsal integument may develop an armature of overlapping plates (Psolus), which may bear spines (Uchinocucumis). Underlying the perisome is a layer of circular muscular fibres, some of which pass into the mesenteries; a second internal set of five simple orpaired bands of longitudinal mus cular fibres are at tached at one end to the radial pieces of the calcareous oral rills , , , ,*- FIG. 35. Spicules of Holothuridea. supply branches Semper.) and (After to the Oral tentacles, a and&, anchor and anchor-plate of Synapta indi- ivirl of tVip nrVipr nrp rt .w, Semper; <, spicule of Chirotlola rtgida,Svm- r , per; rf, Vheel-spicule of Chirodota vitletuii, inserted into the GrSffe; e, spicule of Thyone ctiilensis, Semper; i , f ,-1 n .,,, /. <A i anchors and anchor-plate of Synapta Go- 16 ailUS. deffroyi, Semper; spicule of Rhopalodina lagtni- It is by means of the f rmis ^ Gr "y- longitudinal muscles that the Holothurid, when irritated, effects the discharge of its viscera at the hinder extre mity of its body. In the midst of a circle of tentacles, five or multiples of five in number, is the mouth, which is without dental apparatus. The tentacles vary considerably in shape, and may be cylindrical, shield-shaped, pinnate, or ramified. They serve as organs of touch, of nutrition, and occasionally also of locomotion. The alimentary canal is simple, and usually longer than the body, so as to be two of three times folded on itself; it is attached to the interior surface of the body by mesenteries, and may termi nate in a cloaca. Its walls are composed of an external layer of circular, and an internal layer of longitudinal muscular fibres, and an innermost cellular lining. In com mon with the peritoneal surface of the body, it is ciliated. Two, or more rarely four or five, branched processes of the cloaca, the respiratory trees or ivater-lungs, are ordinarily present, and are connected to the body-wall by a mesentery or by threads (fig. 36). They appear to be excretory in function, water being continually passed into and out of them through the contractile cloaca. Their ulti mate ramifications terminate in minute openings, by means of which they appear to have com munication with the peritoneal cavity. In Echinocucumis the re spiratory trees are only single- branched. In Synapta, in which they do not occur, there are funnel- shaped ciliated bodies attached to the mesentery of the alimentary canal. The respiratory tree occu pying the ventral left interradial space has been observed in many cases to be surrounded by a plexus FIG. 36. Diagrammatic of the pseudhsemal system of vessels, section of a Holothurid. What are termed Cuvierian organs (After Gegenbaur.) . / i i i-i , anus; b. cloaca; c, ahmen- are appendages Of the Cloaca, Wllich, tary canal; d, d, respiratory -ir-nnrrlmrr tr> Mpmrvpr l~Rpi<tpn im t ees; <?, Polian vesicle; /, according ) temper {xeisen im ambulacra! ring; p, tentacles; Archipel der Philippinen, i. pp. 139, *, longitudinal muscle. 1 40), are muscular, and can be used as a means of de fence, being capable of protrusion externally. The main trunks of the pseudhnemal system, which is often exceed ingly complex, are two vessels, one on the dorsal, the other on the ventral face of the intestine, which are con nected with each other- by capillary reticulations. The
calcareous ring surrounding the gullet, already referred to.