550 INVERTEBRATE MORPHOLOGY. 



of the chambered organ. The terminal branches of the radial 

 aboral nerves pass to the integument of the oral surfaces of 

 the arms and to the muscles which unite the various plates, 

 so that the system governs and coordinates the movements of 

 the arms and pinnules as well as of the cirri. The epithelial 

 system, on the other hand, controls the movements of the am- 

 bulacral and oral tentacles, stimulation of it causing move- 

 ment of these structures in the immediate vicinity of the 

 region to which the stimulus is applied. 



Another system of nerve-fibres, consisting of a pericesoph- 

 ageal ring which sends off two branches to each arm, one 

 lying on each side of each of the ambulacral grooves, and 

 which is connected with nerve-fibres passing from the dorsal 

 organ, has been described as occurring, but its significance 

 has not yet been satisfactorily determined. No special sense- 

 organs occur in the Crinoids. 



The reproductive organs are developed for the most part 

 in the pinnules, occasionally a slight development of them 

 appearing in the arms or even in the body proper ; in Holopus 

 alone they are confined to the arms. They consist of tubes 

 lined with germinal epithelium on their inner surfaces and 

 enclosed within a prolongation of the ccelom. They lie be- 

 tween the two coilomic prolongations of the arms already 

 mentioned, and though the reproductive organs are developed 

 only in the pinnules as a rule, nevertheless each genital tube 

 or rachis (Fig. 251, yr) can be traced through the arm to the 

 body, where it terminates in connection with the dorsal organ. 

 In their development indeed they grow out from this organ, 

 and it seems probable that the ova and spermatozoa mother- 

 cells migrate out from it along the rachides to reach maturity 

 in the pinnules. Comparing this with the condition in other 

 Echiuoderma, it seems clear that the so-called dorsal organ 

 of the Crinoids is homologous with the ovoid gland of the 

 other forms. The reproductive elements pass to the exterior 

 by one or two ducts connected with each reproductive mass ; 

 the origin of these ducts is unknown. 



The Crinoids seem to have been closely related to two groups of forms 

 known only as fossils. These were the Cystoids, which appear in the Lower 

 Silurian rocks and die out in the Carboniferous, and the Blastoids, jvhich 



