ASTEROIDEA. 565 



oral nerve ring with its radial trunks lies at the base of the ectoderm 

 cells, which are large and columnar in these tracts, and contain numerous 

 sense cells. They are merely an exaggerated development of the plexus of 

 fibres and ganglion cells present between the ectoderm of the perisoma. 

 The fibres, however, are more numerous and run all in the same direction. 

 There is an eye speck present on the ventral side of the base of the 

 terminal arm-tentacle except in Brisinga. It consists of an aggregation 

 of minute invaginations of ectoderm cells, many of which develop pigment. 

 The blood-vascular system consists of an oral ring with five radial vessels : 

 of an aboral ring which is circum-anal and gives off ten genital vessels in 

 pairs and two intestinal vessels, one on either side of the dorsal extremity 

 of the plexiform organ. These rings and vessels may themselves be plexi- 

 form. The plexiform organ or heart so-called connects the two rings and lies 

 to the inner, i. e. adcentral side of the stone-canal. It is attached dorsally 

 on the central side of the madreporite. A right and left perihaemal space 

 surround the radial vessels ; an outer and inner circular space the oral 

 ring, of which the latter is connected to a perihaemal space inclosing the 

 plexiform organ and stone-canal. Other spaces surround the aboral ring 

 and its vessels. None of these spaces are separated inter se by complete 

 partitions, and it is stated that the inner oral space and the spaces of the 

 genital vessels are connected to the lacunar canals in the perisoma. Feeble 

 contractions have been observed in the intestinal vessels and plexiform organ. 

 The latter contains numerous amoeboid brown cells which also occur in 

 the vascular system generally ; in the water-vascular system, especially in 

 Tiedemann's vesicles, and sparingly in the coelome. They are probably 

 respiratory like the similar cells of Echinoidea. The water-Vascular ring 

 bears four pairs of Tiedemann's vesicles on its inner aspect interradially. 

 The fifth interradius Contains one vesicle and the origin of the stone-canal. 

 This canal has calcareous plates in its walls, is lined with ciliated epithelium, 

 and its cavity is sometimes simple, sometimes broken up into many tubes 

 by internal septa. There is an ampulla at its dorsal end close to the 

 madreporite. This structure is interradially placed, and is sometimes 

 multiple. It is sometimes in connection with one of the basals of the 

 apical system, but is not so at first, nor in some adults> e.g. Zoroaster 

 fulgenS) &c. The radial water- vascular vessels are placed in the axis of the 

 ambulacral groove, immediately ventral to the inner ends of the (sub)- 

 ambulacral ossicles. The tube-feet are arranged in a single row on either 

 side of the vessels. Each foot sends off at its base a canal which, passing 

 through the corresponding pore inclosed by two adjacent (sub)-ambu- 

 lacrals, expands on the dorsal side into an ampulla. These ampullae are 

 absent in Brisinga. The feet are terminated by sucking discs, except in 

 the Astropectinidae, where they are pointed. They are richly supplied by 

 nerves, and gland cells have been detected on their discs. The terminal 



