104 ZOOLOGY. 



rolled m a spiral and press laterally against one another so 

 as to enclose a hermetically closed cavity. " The pinnules 

 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 sliort, 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- 

 lacral 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 pinnulje are con- 

 nected with the radial vessel. Ludwig has also discovered 

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

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

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

 in Asferias. 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 hsemal canal of Echini 

 and Aster i as. 



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 



