546 THE ANIMAL KINGDOM. 



function. In Thyonella gemmatal (Holothurioidea) and Opkiactis wrens 

 (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 (Holotkurioided) ; or from 

 the left of two peritoneal vesicles (Echinoidea, Asteroidea, Ophiuroidea). 

 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 Holothurioidea, 

 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 Holotkurioidea : 

 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 inter- 

 radius, but generally more, and the two sets of structures only communi- 

 cate through the coelome. They are continuously connected in all other 

 Echinoderms. A simple pore is retained by certain Elasipoda among 

 Holothurioidea, 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 Holothurioidea] ; retain 

 its independence (some Asteroidea] ; or fuse with a basal (other Asteroidea, 

 Echinoidea) ; or with an oral (Ophiuroidea). The primary water-pore 



1 Howell, Studies Biol. Lab. Johns Hopkins Univ. iii. (6), i885 ; Foettinger, Archives de Biol. 

 i. 1880. 



