ECHINODERMATA. 183 



of valves so disposed as to prevent any retrograde motion of the 

 blood. At the termination of the aorta there appears to be a 

 second enlargement, to which the name of ventricle has been 

 given, and which is perhaps also capable of contraction, so as 

 to assist in the propulsion of the circulating fluid. The blood 

 of these animals is of a purple colour in the veins, but red in the 

 arterial vessels. 



(224.) We have seen that the tentacula are, from their vascu- 

 larity, 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 neighbourhood of the anal opening, two apertures arc visible, 

 which lead into two long sacculi (jf, j^), the entrance being 

 guarded by muscular fibres (g) : their texture presents transverse 

 and longitudinal striae, and they contract spontaneously even 

 after the animal is dead ; internally they are lined with a mucous 

 membrane. The use of these organs is not precisely known ; 

 Cuvier regarded them as belonging to the generative system, 

 while Delle Chiaje looks upon them as respiratory organs, inter- 

 mediate in structure between the arborescent tubes of Holothuria, 

 and the respiratory vesicles which we shall afterwards find in some 

 of the ANNELIDA. 



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

 allied to the Homogangliate type, we may naturally expect a 

 more complete developement of nervous ganglia than we have yet 

 met with in the class ; and accordingly we find, upon the an- 

 terior part of the oesophagus, two little nervous tubercles (t), 

 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. 



(226.) We are entirely ignorant concerning the mode of repro- 

 duction in these creatures, as no generative apparatus has as yet 

 been distinctly pointed out. 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 which exists 

 in the vicinity of the tail. 



