136 ZOOLOGY. 



rolled in a spiral and press laterally against one another so 

 as to enclose a hermetically closed cavity." The pinnule* 

 are formed of broad flat joints, and are " rolled spirally to- 

 ward the ambulacral channel of the arms when contracted " 

 (Pourtales). The only species yet known is H. Rangii 

 D'Orbigny, from Barbadoes. 



In Antedon (Comatula) the body is at first stalked, but 

 afterward drops off, when it represents the calyx and arms 

 of the ordinary Crinoids. It thus passes through a Rhizo- 

 crinus condition, showing that it is a higher, more recent 

 form. The mouth opens into a short, broad oesophagus, 

 and a wide stomach which makes a turn and a half, ending 

 in the anal cone placed between the base of two of the arms. 

 Within the five triangular plates is a circle of tentacles. 

 From the space between each pair of oral plates the ambu- 

 lacra! grooves radiate to the arms and their branches. H. 

 Ludwig maintains that Antedon possesses a true water- vas- 

 cular system formed on the typical Echinoderm plan ; 

 there being a ring-canal, with radial vessels arising from it. 

 The tentacles of the perisome are connected with the ring- 

 canal, and the tentacles of the arms and pinnulae are con- 

 nected with the radial vessel. Ludwig has also discovered 

 in Antedon a system of blood-vessels ("pseudo-haemal" 

 system) consisting of an oral ring-canal and five vessels 

 radiating from it, which send branches to the tentacles, as 

 in Asterias. He also detected a " dorsal organ," which 

 he, contrary to Perrier and P. H. Carpenter, considers to- 

 be the central organ of the whole system of blood-vessels. 

 Both Ludwig and Carpenter, however, regard it as homolo- 

 gous with the so-called " heart " or haemal canal of Echini 

 and Asterias. 



The nervous system consists of an oral ring with branches 

 extending into the arms. 



The body-cavity extends into the arms, and the ovaries 

 for the most part lie in the cavity of the arms, as in Asterias. 



The internal anatomy of Rhizocrinus has been investi- 

 gated by Ludwig, who finds that it agrees very closely with, 

 that of Antedon. The water-vascular system, nervous sys- 

 tem, alimentary canal and its appendages, have the same 



