ECHINODERMATA 61 



swimming embryos, which pass through different stages 

 and then settle down to assume the adult form. 

 The group is placed in five sections, as follows : 



Asteroidea. The true star-fishes (Figs. 23, etc.), usually 

 five-rayed. (Solaster has twelve or more.) 



Echinoidea. The sea-urchins (Figs. 29, etc.). 

 Holothuroidea. The sea-cucumbers (Fig. 34). 



Crinoidea. The feather-stars (Fig. 27). 



There are about thirty species of Echinoderms on our 

 shores, of which twenty-five or so are to be found in the 

 portion which uncovers by the tide. 



There are. however, some species which are confined 

 to our northern coasts, others to the southern, but it is 

 a poor portion of any coast that does not furnish a dozen. 



In all rock pools where there are bits of loose stone and 

 gravelly or sandy bottom, with their thread-like rays 

 often tangled in the filamentous algae, or among the 

 branches of zoophytes and corallines, may be found numbers 

 of the dainty Little Brittle Star (Amphiura elegans). This 

 one is about an inch across its extended rays, and the central 

 disc is about the size of a small split pea. Its colour is 

 greyish white, sometimes pink. Closely resembling it are 

 one or two others, just differing in the shape of some of 

 the protecting plates. 



In similar situations, but confined to low-tide limit, 

 may often be found the " Thread- rayed star " (Ophiocoma 

 fdiformis). Its disc is about the size of a threepenny piece, 

 and its rays spread out to cover an area of eight or nine 

 inches. It is considered rare, but the fact is that it is often 

 of the colour of its surroundings, and its very slender arms, 

 entangled in masses of Polyzoa, etc., render it very in- 

 conspicuous. 



Still in these situations, but much more abundant in 



