CRINOIDEA. 575 



under the radial water-vascular vessels. In some Palaeocrinoidea they 

 were regularly arranged in two series. Ciliated branched water-tubes 

 depend from the ring and origins of the. radial vessels and open into the 

 coelome. Water-pores, or short tubular canals with a median ciliated 

 dilatation, open into the coelome from the exterior. Water-tubes and 

 pores, taken together, correspond to the madreporic system of other Echi- 

 noderms. In a young larval Antedon one tube and one pore are found the 

 latter in an oral plate -in the interradius to the left of the anus, the one 

 which contains the madreporite in Asteroidea. In an older larva there is 

 a tube and pore in each of the five interradii, a condition persistent in 

 Rhizocrinus lofotensis. The water-tubes are numerous in other adult 

 Crinoids, e.g. thirty in each interradius in Antedon rosacea. The water- 

 pores are few and pierce the orals in Hyocrinus. In Antedon rosacea there 

 are 1500 on the disc. They are relatively few in the anal interradius and 

 in Actinometra are found on the arms and pinnules. Here they open into 

 the genital canal or coelomic space surrounding the genital rhachis. When 

 the disc is covered with plates the pores may be scattered singly or 

 grouped up to the number of twenty in a plate. The radial water-vascular 

 vessels give off alternately to the right and left, in groups of three each, 

 delicate tubular branches, respiratory in function, which form the tentacles 

 homologous with tube-feet. The vessel is slightly dilated on the side 

 opposite to the origins of the tentacles. 



The genital gland appears to consist of a central portion, not always 

 traceable, within the labial blood-vascular plexus from which a branch 

 extends along the arms and pinnules, surrounded by a plexus of blood- 

 vessels, and contained in the genital canal a special offset of the coelome. 

 These structures lie between the subtentacular and coeliac canals. The 

 gland is essentially a tube lined by an epithelium, from which are derived 

 spermatozoa and ova, according to sex. The greater portion is not fertile, 

 and constitutes the rhachis. Fertile portions occur rarely in the disc and 

 arms. In Holopus alone they are always in the arms ; otherwise they are 

 confined to the pinnules, where they form round or elongated masses. The 

 duct in the female is a wide canal, in the male one or two fine canals 

 to each mass. The mode in which the ducts are formed is unknown. 



Sacculi or aggregations of colourless vesicles of unknown function 

 are found in variable numbers at the side of the radial water-vascular 

 vessels, and in Antedon in the walls of the digestive tube. They are absent 

 in Actinometra, where, however, isolated vesicles may occur in the perisome. 

 Two colouring matters giving special absorption bands are found in Crinoids, 

 Pentacrinin from Holopus, Pentacrinus and Metacrinus, and Antedonin 

 from three species of Actinometra. The yellow or rose-red pigment in 

 Antedon gives no bands. The arms of Comatula and some other Crinoidea 

 may become locally deformed by the attacks of the Myzostomidae, a 



