Angular Sea-Cucumber. 145 



blance. The feet with which these ampullae communicate are seen 

 to he provided with sucker-like ends, though not with calcareous 

 terminal supports, as in the Echinoidea and Holothurioidea. In 

 the three genera of Asteriae, Astropecten, Ctenodiscus and Luidia, 

 the locomotor feet end by conical and not by sucker-like termina- 

 tions; and with this point of inferiority is correlated also the 

 absence of an anus. In the Ophiuridae, the digestive tract is a 

 simple coecal sac, bulging somewhat like the true stomach of the 

 Cross-fish radially, but not prolonged into the interior of the arms, 

 as in all the Asteriae including the three aproctous genera just 

 mentioned and Brisinga ; and with this increase of inferiority in. 

 the digestive apparatus, an increase of inferiority in the locomotor is 

 found to correspond, as the feet are devoid not only of true suckers 

 but also of amj)ullae. It is in the Asteroidea alone that the nerve- 

 system lies externally to the ambulacral plates, which form thus an 

 internal skeleton,^ absent in the Crinoidea, and existing in the 

 Echinoidea and, possibly, in the Holothurioidea also, only as ru- 

 diments. The nerve-cord is further protected in Ophiurae by a 

 row of dermal ventral scutes. 



For a figure of the digestive tract, see pi. x. infra. 

 For figures of the locomotor feet in Ophiurae, see Sars, Norges 

 Echinodermer, 1861, tab, i., fig, 1-5, 



46. Angular Sea-Cucumber {Cucumaria Pentactes), 



Forbes, 



Prepared so as to show the external characters of the class Holothurioidea ; and 

 the traces of a bilateral symmetiy, the co-existence of which with the more obvious 

 appearance of a radial arrangement is very well seen in these Echinodermata. 



The five rows of ambulacral feet and the ten arborescent circum- 

 oral tentacles, which are merely modifications of ambulacral feet, 

 give the Sea-Cucumbers at first sight a very markedly radiate 

 appearance. But upon looking closely at these structures, both 

 will be seen to admit of being divided into two bilaterally symme- 

 trical halves, each of which is composed of heteronomous elements. 

 The five rows of ambulacral feet are seen to fall naturally into a 

 ventrally placed trivium, the feet in each of the rays of which 

 are more numerous, more perfectly developed, and more regularly 



L 



