3270 



ECHINODERMS 



The food of crinoids consists chiefly of foraminifera, diatoms, and the adults of 

 small and the larvae of larger crustaceans. Crinoids themselves form food for fish, 

 though nowadays their place seems to be taken by the brittle stars and an occasional 

 sea urchin. As protection against such attacks, some crinoids have been provided 

 with spines either as movable processes from the plates of the test, as in Dorycrinus 

 from the Carboniferous of North America, or, very rarely, movably attached like 

 the spines of a sea urchin, as in Hystricrinus from the Devonian of the same 

 country. Parasites, however, find crinoids an easy and almost unresisting prey. A 

 suctorial crustacean, eggs and all, has been found in the body cavity, while a 



decapod crustacean 

 occasionally inhabits 

 the intestinal tube. 

 The annexed figures 

 represent the cysts 

 formed by the crinoid 

 in response to the 

 irritation set up by 

 the presence of a 

 parasitic worm, in 

 which cysts it takes 

 up its abode. There 

 are also worms that 

 bore into the stems 

 as well as boring 

 sponges, and coral, 



that affix themselves to the stem. The crinoid generally makes some attempt to 

 overwhelm these intruders by the rapid deposition of the calcareous skeletal sub- 

 stance; so that in the rocks greatly thickened stem fragments are found inclosing 

 the remains of corals, brachiopods, etc. 



SWEUJNGS IN THE PINNULES OP CRINOIDS PRODUCED BY A 



PARASITE (twice natural size). 



THE BLASTOIDS Class Blastoidea 



The Blastoidea constitute a compact group, pretty clearly marked off from both 

 Cystidea and Crinoidea, which they resemble in the upward position of the mouth 

 and the generally fixed habit. The chief characteristic that separates blastoids 

 from other echinoderins is the presence of an elongate plate, the lancet plate, under- 

 lying the ambulacrum and pierced by a canal supposed to have contained the radial 

 water vessel. These five canals meet in a circular canal round the mouth, but 

 there is no evidence that they were connected with tube feet as in other echino- 

 derms. Each side of each ambulacrum was lined by a row of delicate, unbranched 

 arms; and the food grooves of these arms passed to a single groove running down 

 the middle of the surface of the ambulacrum, and these five grooves then passed up 

 to the mouth. 



The most interesting structures in the Blastoidea are the hydrospires. In such 

 a form as Pentremites there are five openings (spiracles) round the mouth, placed 



