! 



SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 143 



ordinarily plentiful that there is no reason why this should 

 not be done. The contrast between the regular urchins 

 with their strong shells, uniform coating of spines, and 

 well-developed tube-feet, and the heart-urchins with their 

 fragile shells, on which both spines and tube-feet are dis- 

 tributed in so complex a fashion, and which have lost the 

 primitive radiate symmetry, is so striking, and so intimately 

 related to the different modes of life, that it is worth 

 careful study. A great part of the interest attached to 

 the Echinoderma is due to the fact that the members of 

 the group show adaptations to many different kinds of life, 

 while retaining those well-defined characters which make 

 the group such a compact one. In many cases the structural 

 adaptations to particular habitats are difficult to study, but 

 in the heart-urchins they are fairly obvious, and intensely 

 interesting. Between tide-marks EcTiinocardium cordatum 

 is the only heart-urchin likely to be found in the living 

 condition ; but on the beach after storms one at times finds 

 the purple heart-urchin (Spatangus purpureus). The differ- 

 ences between it and Echinocardinm are not very striking 

 apart from colour. The most noticeable difference is perhaps 

 the fact that in Spatangus certain of the spines are very 

 long, strong, and curved a difference probably associated 

 with the fact that the animal lives in coarser material 

 (coarse sand or gravel) than Ediinocardium. 



The next group of Echinoderms the Holothurians, or 

 sea-cucumbers is very poorly represented on the East 

 Tloast, at any rate in shallow water, though, indeed, in any 

 case the majority occur beyond tide-marks. For the sake 

 of completeness we may describe a typical form, such as 

 Cucumaria ladea, which does occur between tide-marks 

 occasionally. It is a little creature, about an inch long, 

 with a cylindrical body, and a tough skin of white or brown 

 colour. The form is strikingly different from that of other 

 Echinoderms, for it is characteristic of the Holothurians 

 that their radial symmetry is not obvious, most of them 

 being of worm-like form, and showing more or less distinct 

 bilateral symmetry. If you obtain Cucumaria in the living 

 active condition, you will see it protrude at one end of the 

 body a beautiful crown of ten branched tentacles (te in 

 Fig. 46). At the other end of the body is the anus, and 



