In Thyonella gemmatal (Holothnrioidea) and Ophiactis virens (Ophiuroidea), there are corpuscles tinted with haemoglobin 1. The organ cannot be considered as excretory. It is connected with an aboral vascular ring in Asteroidea and Ophiuroidea, groups in which rings as well as vessels are sometimes plexiform. The water-vascular system is a specialised portion of the coelome developed from the archenteron as a vesicle independent of the peritoneal vesicles (Crinoidea); or as a common vaso-peritoneal vesicle subsequently divided (Holothnrioidea); or from the left of two peritoneal vesicles (Echinoidea, Asteroidea, Ophiuroided). In the last-named group a water-vascular vesicle which aborts is derived also from the right peritoneal vesicle. The embryonic water-vascular vesicle, however formed, always lies on the left side. It surrounds the oesophagus at a later stage, and thus becomes a ring from which arise five caeca. These caeca lengthen out into the five radial water-vascular vessels; each one gives origin to a terminal azygos process, the tentacle of Echinoidea and Asteroidea as well as to paired lateral processes, the tube feet or pedicels. The processes are hollow, and extend into and raise the integument.
The tube feet are variously modified in the different classes, and often become enlarged into organs of respiration or touch. At their inner ends they are often connected to a dilatation or ampulla. The ring itself gives origin to circumoral tentacles in the Holothnrioidea, and it is frequently provided with one or more dilatations or Polian vesicles depending into the coelome. It is also connected to the exterior by a water-tube and pore. The pore lies in the larva anteriorly in the median dorsal line or interradius, a position retained in Holothnrioidea: but in Echinoidea, Asteroidea and Ophiuroidea, owing to the remarkable difference between the dorsal and ventral surface of the larva and adult, the pore shifts either actinally (to the left) or abactinally (to the right), and lies in the interradius which originally corresponds to the anterior extremity, i. e. the one in which the circles of apical and oral plates close. It may be supposed that the first formed pore in Crinoidea indicates the homologous interradius. In all Crinoidea, however, there are formed, during growth at least, five water-tubes and five pores, one in each interradius, but generally more, and the two sets of structures only communicate through the coelome.
They are continuously connected in all other Echinoderms. A simple pore is retained by certain Elasipoda among Holothnrioidea, and some Ophiuroidea, but in others the aperture is usually closed by a calcareous pore plate or madreporite. The madreporite and water-tube, stone canal or madreporic tube, are rarely multiple. The former may be withdrawn into the coelome (most Holothnrioidea); retain its independence (some Asteroidea); or fuse with a basal (other Asteroidea, Echinoidea); or with an oral (Ophiuroidea). The primary water-pore pierces an oral in Crinoidea as do others formed later. The epithelium of the water-vascular system is ciliated partially or throughout. The coelome is derived invariably from a right and left peritoneal vesicle, either separate outgrowths of the archenteron or split off from a common outgrowth (Holothurioidea). The walls of the two outgrowths fuse round the archenteron, and the dorsal fused portion persists as the mesentery, already mentioned, while the ventral portion is absorbed.
The coelome is large, its wall generally ciliated, its cavity filled by a liquid of about the same specific gravity as sea-water, and containing, where it has been examined, but little proteid material and relatively few corpuscles.
1 Howell, Studies Biol. Lab. Johns Hopkins Univ. iii. (6), 1885; Foettinger, Archives de Biol. i. 1880.
The larval mouth and oesophagus are carried on into the adult in Holothurioidea and Ophiuroidea, but in Echinoidea and Asteroidea the oesophagus of the adult is a new formation, growing out from the archenteron to the left side, that to which the mouth is said to be transported. The larval anus is a persistent gastrula mouth. It is retained in Holothurioidea, closed permanently in Ophiuroidea, and a few Asteroidea, said to be retained in Echinoidea and other Asteroidea, but in some instances it certainly closes and a new anus is formed subsequently. The digestive tract is typically a spirally coiled tube. The oesophagus lies in the madreporic interradius, and if the Echinoderm be regarded from the oral surface, the tract passes to the right, and the anus is situated in the fourth interradius in Holothurioidea and Echinoidea, but in the fifth, i. e. the one adjoining the madreporic, interradius in Asteroidea and Crinoidea. The tract is saccular in its first section in Asteroidea, and wholly saccular in Ophiuroidea. It is generally ciliated except in Holothurioidea. There are radial glandular caeca in Asteroidea, and to the anal end of the intestine there are appended, as also in many Holothurioidea, two caeca.
Respiration is carried on by external prolongations of the coelome (dermal branchiae of Asteroidea, peristomal gills of some Echinoidea); by modified tube feet (many Echinoidea, tentacles of Crinoidea); or by the expanded anal end of the intestine with its two appended caeca or respiratory trees (many Holothurioidea). In all cases, however, the tube feet must have a limited respiratory function, and sea-water diffuse through the walls of the stone canal and water-vascular vessels into the coelome. There is no excretory organ.
The sexes are separate. The Ophiuroid, Amphiura squamata, is said, however, to be hermaphrodite. The genital glands are more or less branched interradial caeca, single in Holothurioidea, multiple in other Echinoderms. Details vary in the different groups. Impregnation is either external, or the spermatozoa pass from the water into the female ducts. A few forms in all groups except Crinoidea are viviparous. Development is then abbreviated and takes place internally in the genital bursae (Ophiuroidea) or the coelome (a Chirodota, Phyllophorus urna among Holothurioided); or else externally in depressed ambulacra protected by the spines (certain Echinoided); among the dorsal spines or paxillae (the Asteroid Lepty'chaster); among the dorsal ambulacral processes (the Holothurioid Cladodactyla croccd); or in special marsupia formed by stalked plates (the Holothurioid Psoitis ephippiger), or by a soft skin (the Asteroids Pteraster and Hymenaster, and Echinoid Anochanus). The Asteroids, Echinaster Sarsii and Asterias Mtilleri bend their arms ventrally and so protect the ova. The female in most of these instances is distinguishable from the male.