HOMOOANGLIATA. 213 



(552.) We have seen that the tentaciila are, from their vascularity, 

 well adapted to fulfil the office of a respiratory apparatus ; but it may 

 be presumed that they are not the only agents by which respiration is 

 accomplished. Upon the outer surface of the body, in the neighbour- 

 hood of the anal opening, two apertures are visible, which lead into two 

 long sacculi (/, ^>), the entrance being guarded by muscular fibres (g) : 

 their texture presents transverse and longitudinal striae ; and they con- 

 tract spontaneously, even after the animal is dead ; internally they are 

 lined with a mucous membrane. The use of these organs is not pre- 

 cisely known ; Cuvier regarded them as belonging to the generative 

 system, while Delle Chiaje looks upon them as respiratory organs. 



(553.) In this elevated form of the Echinodermata, so nearly allied 

 to the Homogangliate type, we may naturally expect a more complete 

 development of nervous ganglia than we have yet met with in the class ; 

 and accordingly we find, upon the anterior part of the oasophagus, two 

 little nervous tubercles (i) from which nervous filaments issue to be 

 distributed to different parts of the body ; one of these, in particular, 

 may be traced along the whole length of the intestine, from the mouth 

 to the anus. 



(554.) We are entirely ignorant concerning the mode of reproduction 

 in these creatures. Nevertheless, at certain seasons of the year, on 

 opening the visceral cavity it is found to be filled with a fluid of a 

 reddish tint, in which thousands of minute white bodies resembling 

 millet- seeds are seen to float : should these be ova, they are probably 

 expelled through an orifice that exists in the vicinity of the tail. 



CHAPTER IX. 



HOMOaANGLIATA (Owen). 

 ARTICULATA (Cuv.). ANNULOSA (MacLeay). 



(555.) THE next great division of the animal kingdom includes an 

 immense number of living beings, adapted by their conformation to 

 exist under a far greater variety of circumstances than any which we 

 have hitherto had an opportunity of examining, all of which are ob- 

 viously only adapted to an aquatic life, and accordingly are invariably 

 found either to inhabit the waters around us, or to be immersed in the 

 juices of living animals upon which they subsist. Even the Echino- 

 dermata are too imperfect in their construction to admit of their en- 

 joying a terrestrial existence, inasmuch as, possessing no nervous cen- 

 tres adequate to give force and precision to their movements, they are 

 incapable of possessing external limbs endowed with sufficient power 



