546 ECHINODERMATA ECHINOIDEA chap. 



amoebocytes wander. In Uchinarachnius these partitions are 

 arranged in groups, each group radiating from a common centre. 



The main peculiarities in the structure of Uchinarachnius are 

 comprehensible when the species is viewed from above in its 

 normal environment. It is found in comparatively shallow 

 water on a sandy bottom, and normally is nearly but not quite 

 buried in the sand. It might thus be overturned by the 

 force of the waves and currents, and it is protected against this 

 fate by its flattened shape. This shape, however, necessitates 

 some kind of support for the upper part of the test, and this is 

 provided by the internal partitions. 



In order to view the internal anatomy of the " Sand-dollar," it 

 is necessary carefully to pick away the dorsal surface of the shell 

 piece by piece. In this way the whole course of the alimentary 

 canal is exposed ; as in Echinus esctolentus it can be seen to issue 

 from the upper surface of Aristotle's lantern. It then bends 

 sharply to the left, and makes a complete circle round the edge 

 of the disc ; this portion is the stomach, and is considerably in- 

 iiated and accompanied by a " siphon." It then bends sharply 

 back on itself, but only goes half way round ; when it reaches the 

 posterior interradius it ends in the anus (Fig. 244). 



Aristotle's lantern is greatly simplified as compared with its 

 condition in the Eegular Urchins. Both rotulae and compasses 

 are absent ; the jaws are sharply bent on themselves, and their 

 appearance gives one the impression that they have shared in 

 the process of compression which the test as a whole has under- 

 gone, and have thus become bent. The teeth are nearly horizon- 

 tal, and they actually articulate with the auriculae, which, as 

 in Cidaridae, consist of disconnected pillars and spring from 

 the plates of the interradius. Each pillar is fused with the 

 adjacent one belonging to the next radius, so that the system 

 which in Echinus consists of five radial arches here consists of 

 five interradial pillars. Aristotle's lantern has lost its respira- 

 tory function and apparently its masticatory function as well, for 

 the teeth are used as spades to shovel into the mouth the sand 

 mixed with organic detritus and small organisms on which the 

 animal lives. 



The water-vascular system is highly modified. There are 

 two sharply marked kinds of tube-feet — (a) the respiratory tube- 

 feet, (&) the locomotor tube-feet. Both kinds are terminated by 



