ECHINOIDEA. 235 



and have the power of climbing steep slopes, by means of their 

 powerful suckers. They also use the large spines for locomo- 

 tion. In the genus Cidaris, in which the spines are very large, 

 the feet are not important in locomotion and have feeble suckers. 

 Hence this genus prefers deep water in which it is not rolled 

 about. They frequently live in cavities, which they hollow out 

 of quite hard rock by mechanical action of their teeth or other- 

 wise. The Exocydica on the other hand for the most part 

 live in sand, and the locomotive tube-feet are relatively feebly 

 developed. In Echinocardium cordatum, which lives buried about 

 eight inches deep in sand, the tube-feet of the anterior ambulacrum 

 near the apex are very long, several times the length of the animal. 

 They project up through the sand through a hole above the 

 apex and end in frilled suckers. They catch food, shrink back 

 through the hole, and hand it to the buccal tentacles.* 



There can be little doubt that the affinities of the Echinoidea 

 are with the Asteroids. The general plan of structure and the 

 relations of the chief systems of organs are the same in the two 

 classes. The anatomical differences are small and relate to 

 comparatively unimportant features, such as the structure of 

 the alimentary canal and the calcareous covering of the body. 

 The most important difference is the closure of the ambulacral 

 groove and its conversion into the epineural canal, so that the 

 ectoneural central nervous system is placed in the wall of a 

 closed canal as it is in Chordates, but this peculiarity is shared 

 by Ophiurids. The differences in external form, though con- 

 siderable at the first glance, are much diminished on a close 

 inspection. If the oro-anal axis of a pentagonal Asteroid such as 

 Asterina be elongated and the antambulacral surface reduced 

 we get the body-form of a typical sea-urchin. The madreporite 

 in being aboral and interraclial occupies a similar position in 

 the two classes, and in both Asteroids and regular Echinoids the 

 anus though close to the aboral pole is always slightly excentric. 

 This position of the anus is highly significant not only as in- 

 dicating the fundamental asymmetry of the body but also be- 

 cause it is identical or nearly identical in the two groups. In 

 Asteroids it is placed in the interradius adjacent to that of the 

 madreporite (Fig. 122) ; in the regular Echinoids it is either 



* An observation of the late Dr. Robertson of Cumbrae communicated 

 by Dr. MacBride. 



