RESPIRATORY AND EXCRETORY SYSTEMS. 239 



near the upper end of the lantern. This gives off five inter- 

 radial transparent vesicles, and five radial vessels which run 

 down the sides of the lantern and up each ambulacral area. 

 Each radial vessel gives off numerous lateral branches, 

 which communicate with the internal ampullae and thence 

 with the external tube feet. When the tube feet are made 

 tense with fluid, they extend beyond the limit of the spines, 

 and are attached to the surface of the rock over which the 

 sea urchin slowly drags itself. The sucker at the tip of each 

 tube foot bears small calcareous plates regularly arranged, 

 indeed there is hardly any part of an Echinoderm in which 

 lime may not be deposited. Before bending upwards 

 from the base of the lantern, each radial vessel gives off a 

 branch to two large tentacle-like tube feet without attach- 

 ing discs. The five pairs lie near the mouth, and are 

 sensitive. 



The Blood Vascular System is not readily traced, and there is un- 

 certainty as to many points. Along the stone canal lies an enigmatical 

 structure, to which such names as " plexiform organ," " ovoid gland," 

 "dorsal organ," and "heart" are given. Its structure is like that of 

 the smaller glandular enlargements found on the vascular system. 

 According to some, it gives origin to some of the amceboid cells of the 

 body cavity fluid. It is connected superiorly with the five genital 

 organs, inferiorly with a circular vessel surrounding the pharynx at the 

 top of the lantern, within and beneath the water ring. This vascular 

 ring seems to be connected, by branches at least, with the five pockets 

 of the water ring. A distinct vessel arises from the ring and runs along 

 the inner or ventral surface of the intestine, while another on the 

 opposite side seems to originate from capillaries. It is likely enough 

 that there may be radial blood vessels or spaces in the ambulacral areas. 

 The fluid cannot be distinguished from that of the body cavity ; it 

 contains corpuscles, some of which have pigment. 



Respiratory and Excretory Systems. 



On the area round about the mouth there are ten hollow 

 outgrowths, which resemble the skin gills of starfishes. As 

 already mentioned, the pigmented cells of the body cavity 

 fluid seem able to absorb oxygen. The water vascular 

 system plays here a very important part in respiration. 

 Waste products seem simply to accumulate in the tissues, 

 but Hartog maintains that the water vascular system helps 

 in excretion. 



