184 NERVOUS SYSTEM. 



The echinoderma, like the acalepha, are all inhabitants of 

 the sea, and, as in other aquatic animals, they present a de- 

 velopment of the muscular and nervous systems, and of all 

 the other organs of relation, inferior to that of animals of a 

 corresponding grade, inhabiting the land ; their nervous fila- 

 ments are also less white and opaque, less firm in texture, 

 and less obvious than in the land animals. , The distribution 

 of the nervous system has been long known in several ge- 

 nera of this class, and it presents the same circular disposi- 

 tion as in the acalepha and other cyclo-neurose animals. In 

 the asterias (Fig. 81. B,) which has but one opening of the 

 alimentary cavity (e,) there is a small white opaque nervous 

 chord (,) passing round the mouth. From this circular 

 chord a nerve (c, c,) is given off to each of the radiating di- 

 visions of the body, which passes along the middle of the 

 lower parietes between the ambulacra, and gives filaments 

 to the muscular suckers and other parts of the rays. Mi- 

 nute ganglia (b,) are observed at the points where these ra- 

 diating nerves originate, and from each of these ganglia 

 two nerves (d,) extend obliquely upwards along the sides of 

 the stomach, and are confined in their distribution chiefly to 

 the parts contained within the central disk of the body. A 

 similar nervous chord is seen around the oesophagus of the 

 echinus, which sends delicate white filaments to the compli- 

 cated muscular and sensitive apparatus of the mouth ; other 

 nerves are seen extending upwards from the same oesophageal 

 ring, along the course of the vessels in the interior of the ab- 

 dominal cavity. In the holothuria, where the axis of the 

 body is greatly lengthened, and the animal reclines and 

 moves on one side of the trunk, like the higher classes, 

 where the calcareous shell is wanting, and the muscular sys- 

 tem is most distinct and powerful, the nervous system is 

 extensively distributed, and begins to manifest an approach 

 to the helminthoid type. Interior to the osseous apparatus 

 of the mouth is a white nervous ring around the oesophagus, 

 from which nerves pass outwards to the large ramified ten- 

 tacula around the mouth, and others extend upwards along 

 the course of the eight strong longitudinal muscular bands. 

 Fine white filaments are likewise seen passing inwards to the 

 stomach and alimentary apparatus. In the siponculus, which 

 is closely allied to the holothuria in internal structure and living 



