STARFISH. 203 



thuroids are nearer to the Echinoids than to any of the 

 others, {b) The Echinoids are linked by transition types to 

 the extinct cystoids. {c) There are direct affinities between 

 Echinoids and Asteroids, and this further connection that 

 the Asteroids are linked to the Cystoids. {d) The Ophiuroids 

 are very closely allied to the Asteroids, if indeed the two 

 sets do not form one class. («) The Blastoids and Crinoids 

 are related to one another, and also to the Cystoids, 

 which thus form a central class. 



In our survey of the classes, we shall begin with the star- 

 fish-like forms which are perhaps most familiar ; with these 

 the brittle-stars will be briefly contrasted ; we shall then 

 describe the sea-urchins and sea-cucumbers, and finish 

 with the Crinoids. The general characters of each class 

 may be read from the synoptic table at the end. 



Class AsTEROiDEA. Starfish. 



The description applies especially to Asterias or Aster- 

 acanihion rubens. 



Form. — The common five-rayed starfish (Asterias or 

 Asteracanthion -rubens) is sometimes seen in shore pools 

 exposed at low water, but its haunts are on the floor of the 

 sea at greater depths. There it moves about sluggishly in 

 any direction by means of the tube-feet of the water-vasculaj 

 system, to be afterwards described. Each arm bears a deep 

 ventral groove in which the tube-feet are lodged. The 

 mouth is in the middle of the ventral surface, the food-canal 

 ends about the centre of the dorsal disc. With this flat, 

 five-rayed form, the 11-13 rayed sun-star (Solaster), the 

 pincushion -like Goniaster, and the pentagonal pancake-like 

 Falmipes, should be contrasted. 



Integument. — {a) The body is covered by a ciliated 

 ectoderm. This includes supporting, glandular, and 

 sensory cells, and beneath it there is a network of 

 nerve fibrils with ganglionic cells. 

 {b) The mesodermic part of the integument consists of 

 two layers with irregular channels between them. 

 In the outer of these layers are formed all the limy 

 structures, except one set (the ambulacral ossicles). 



