xvni BLOOD SYSTEM CLASSIFICATION 529 



as " sea eggs " in the West Indian Islands. One female 

 Echinus esculentus will produce 20,000,000 eggs in a season. 



The so-called blood system is more distinctly developed in 

 Echinoiclea than in Asteroidea and Ophiuroidea. There is an oral 

 ring of lymphoid tissue surrounding the oesophagus below the 

 water-vascular ring. From this are given off two strands, the 

 so-called "dorsal" (Fig. 231, b.v), and "ventral" vessels (Fig. 

 234, 16), which run along the two opposite sides of the stomach 

 or first coil of the alimentary canal. The position of these 

 strands suggests that like the lacteals of the human intestine 

 they are channels along which the products of digestion exude 

 from the stomach. The dorsal strand is situated on the same 

 side as the genital stolon, and from it branches are given off 

 which ramify on the surface of the stolon, on account of which 

 this organ, as in Asteroidea, was at one time regarded as a " heart," 

 but the distinction of the stolon from the strands is easily made 

 out. An aboral ring enclosing the genital rachis lies embedded 

 in the septum dividing the aboral sinus (Fig. 234, 20) from the 

 general coelom. 



Classification of Echinoidea. 



The Echinoidea are sharply divided into three main orders, 

 which differ from each other profoundly in their habits and 

 structure. These are: (1) The Endocyclica or Eegular 

 Urchins, of which the species just described may be taken as the 

 type. (2) The Clypeastkoidea or Cake-urchins, which are of 

 extremely flattened form, and in which the periproct is shifted 

 from the apical pole so that it is no longer surrounded by the 

 genital plates, while some of the tube-feet of the dorsal surface 

 are flattened so as to serve as gills. (3) The Spatangoidea or 

 Heart-urchins, in which the outline is oval : the periproct is 

 shifted, as in the Cake-urchins, and the dorsal tube-feet are 

 similarly modified ; but the Heart-urchins have totally lost 

 Aristotle's lantern, whilst the Cake-urchins' have retained it. 

 This strongly-marked cleavage of the group was primarily due, as 

 in all such cases, to the adoption of different habits by different 

 members of the same group. Were we to term the three orders 

 Bock-urchins, Sand-urchins, and Burrowing-urchins, it would not 

 be entirely true, for secondary invasions of the other's territory on 



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