VASCULAR SYSTEM OF ECHINODERMA. 217 



passes into the rays. Thus in the Asterida and in Brisinga it 

 extends through the arms. The same thing happens in the 

 Crinoi'daj but in them the canals are narrower. In each arm it can 

 be divided into three parts, which are connected with special 

 divisions of the coelom of the disc. This latter portion is separated 

 into several divisions by connective bands, which here and there 

 form membranous tracts; they communicate with one another at 

 certain points, and at other points pass into the canals. In the 

 Echinoida and Holothuroida, where the organism is more concen- 

 trated, the ccelom is more simple. In the former, however, the 

 mesenteric filaments, and still more the calcified pillars and columns, 

 which traverse the coelom of the Clypeastridge, remind us of divisions 

 into separate parts ; several such spaces are also marked off in the 

 coelom of the Holothuroi'da. In the Asterida and Echinida, as well 

 as in the Holothui-oi'da, the parietal and visceral tracts of the coelom 

 have been observed to be provided with cilia. The contents of 

 the coelom appear to be the same in character as the blood, so that 

 in it we have to recognise a portion of the blood-cavity. In some 

 cases communications with the exterior have been definitely observed 

 (Orinoida) ; as also communications with the water- vascular system 

 (Crinoida, Holothuroi'da). The first set are due to numerous 

 canaliculi, which traverse the interradii of the perisome, and open by 

 the so-called calycine pores. 



Vascular System. 



Blood-Vessels. 



§ 176. 



The nutrient fluid in the Echinoderma is a clear, or slightly 

 opalescent fluid, which is seldom thick or even coloured, and which 

 is very probably mixed with the water which is taken in from the 

 exterior. The form-elements in this fluid are simple cells. 



The blood-cavity is formed in the flrst place by a special system of 

 canals, but also by the coelom, which is probably connected with a 

 third cavitary system, the system of so-called water- vessels. Owing 

 to the uncertainty of our knowledge of this Vascular System, that 

 is, of its mutual relations and connections, it is as yet impossible to 

 make any generalisation which will hold for all the divisions ; 

 although indeed remarkable progress has lately been made in our 

 knowledge of this part of the anatomy of the Echinoderma. But 

 from the similarity of construction of these canals and spaces we 

 may suppose that a connection between them does really exist. 



The close association of the haemal system and the nerve-tracts 

 may, howevei', be regarded as a general arrangement. A blood- 

 vascular trunk accompanies each radial nerve-trunk, and is continued 



