MUSCULAR SYSTEM. 135 



cular systems "are already obviously developed, and eyes are 

 observed on the asterias. The segments of many of the 

 stellerida are connected together and moved by muscular 

 bands, which are distinctly fibrous, and both longitudinal 

 and circular fibres are seen on the highly irritable and con- 

 tractile feet, which extend from the ambulacra. The same 

 muscular structure is seen in the long tubular prehensile feet 

 of the echinida, where the segments of the trunk are im- 

 moveably united by harmonic sutures. Strong adductor 

 and abductor muscles are seen attached to the moveable 

 pieces of the jaws in many of these species, and strong 

 muscular fasciculi pass from around the base of the external 

 spines to the periphery of the tubercles on which they move. 

 In the naked holothurige, the tough, exterior, coriaceous 

 covering is . highly irritable and contractile ; these animals 

 are provided with five crowded longitudinal rows of the 

 usual tubular, muscular, prehensile feet extended like those 

 of other echinoderma by the injection of water into their 

 median cylindrical cavity ; their dental apparatus is provided 

 with its muscular bands, and the five larger of these osseous 

 plates around the mouth give attachment to five strong lon- 

 gitudinal muscular bands which extend beneath the skin to 

 the posterior extremity of the body. 



THIRD SECTION. 



Muscular System of the Diplo-Neurose, or Articulated 

 Classes. 



The development and energy of the muscular system are 

 greater in the articulated than in the radiated, or even the 

 molluscous classes, which corresponds with the greater ex- 

 tent of their respiration, and with the higher condition of all 

 their other organs of relation. In the subcutaneous mus- 

 cular tunic of the nematoid entozoa, we already perceive the 

 longitudinal and transverse short interrupted filements, which 

 are more symmetrically arranged into bands and muscles, 

 where the segments of the trunk are more consolidated and 



