262 PHYLUM ECHINODERMA. 



arms. They agree with starfishes in being free, in having 

 radially disposed gonads, in having the tube-feet restricted to 

 the under surface, and in other features. 



The body of a brittle-star differs from that of a star- 

 fish in the abruptness with which the arms spring from 

 the central disc (cf. Brisingd). These arms are muscular, 

 and useful in wriggling and clambering ; they do not con- 

 tain outgrowths of the gut, nor reproductive organs. 

 Moreover, there is no ambulacral groove, and the tube-feet 

 which project on the sides are usually very small. They 

 are often of locomotor service, adhering even to vertical 

 surfaces, but in many cases they seem to be only sensory. 

 Each segment of the arm includes a central "vertebral 

 ossicle," with four plates forming a tube round about it. 

 There is a complete oral skeleton. The madreporic plate 

 is situated on the ventral surface, usually on one of the 

 plates around the mouth. The food canal ends blindly. 



Some brittle-stars have small luminescent glands, e.g. 

 Amphiura squamata. The reproductive organs lie in 

 pairs between the arms, and open into pockets or bursse 

 formed from inturnings of the skin, which communicate 

 with the exterior by slits opening at the bases of the arms. 

 Water currents pa'ss in and out of these pockets, which 

 probably have both respiratory and excretory functions. 



The free-swimming larva is a Pluteus, very like that of 

 Echinoids (see Fig. 131). 



Ophiuroids are first found in Silurian strata. 



The Ophiuroids are usually classified according to the characters of 

 their ossicles and covering plates. Some common genera are Ophiothrix, 

 Ophiocoma, Ophiopholis, Ophiura. In the deep-water Astrophyton and 

 Gorgonocephalus the arms are repeatedly branched. In Astronyx 

 loveni, often caught in the trawl off the north coast of Britain, the disc 

 is relatively large and soft and the arms very long. In the extinct 

 Lysophiurae there is an ambulacral groove. 



Gass ECHINOIDEA. Sea-Urchins, e.g. the common 

 Echinus esculentus 



* Echinoderms with the body covered by rows of plates, 

 usually in vertical series and forming an inflexible test ; the 

 shape of the majority approaches a sphere, but some are pin- 



