ALIMENTARY CANAL. 675 



The terminal anal extremity of the alimentary canal does not differ materi- 

 ally in some Clypeastridae from the rest of the canal (PL XXVIII. f. a ; PL 

 XXIX. lower fig.); in the Scutellidae it is often remarkably slender (PL 

 XXXI. f. 2). The alimentary canal is separated by pillars or by continuous 

 walls from the actinal cavity, and also from the ambulacral system ; the 

 arrangement and structure of these walls and pillars giving excellent features 

 to distinguish generic groups, when taken in connection with the marginal 

 pillars and walls uniting the two floors. In Echinocyamus the marginal par- 

 titions radiate from the ambitus to the actinostome {PI. XIII f. 7), and 

 there are no walls separating the digestive cavity from the ambulacral system 

 or the actinal cavity. In the Laganidae (PL XXXIII f. .;, 4 ; PL XIII'. f. 

 10, ll) the marginal supports form disconnected walls, concentric with the 

 outer edge, without separating the alimentary canal from the actinal cavity. 

 In the Scutellidae we find the marginal supports usually in the shape of 

 radiating pillars (PL XHP.f. 3 ; PL XI P. f. 6; PL XHP.f. 1, 2, ■>). and in 

 some genera the first trace of small pillars (PL XII". f. 4) appears (PL XIII d . 

 f. 1), between the marginal pillars and the auricles. This system of pillars 

 is more and more complicated in some of the Scutellidae (PL XI d . f. 

 3), Encopidae (PL XIP.f. s, 4) and Echinanthidae, till it finally entirely sep- 

 arates the alimentary canal from the remaining internal cavity of the test 

 {PI. XP.f. 2, 4 ; PL XP.f. s, 4 ; PL XXVIII f. 2). 



The mesenteries which hold in place the alimentary canal are few in num- 

 ber, generally reduced to a simple chord following the alimentary canal, as in 

 the abactinal part of the alimentary canal of the Desmosticha (PI. XXV 111. 

 f. 1 ; PL XXX. upper fig.), and in this case it follows nearly the course of 

 the main vessel following the alimentary canal. 



In Desmosticha the alimentary canal is filled with particles of alga?, those 

 towards the anal part are formed into pellets (PI. XX VIII. f. 5; PL 

 XXVII f. 1, s), and as such pass out of the anal opening, while nearer the 

 actinostome the contents form a homogeneous mass of minutely ground par- 

 ticles of seaweed and the like. In the Clypeastroids the alimentary canal is, 

 as in the Petalosticha, filled with sand in spite of the presence of jaws and 

 teeth. The majority of Clypeastroids live upon sandy shores, and use their 

 jaws simply to force in the sand by a process of shovelling ; they do not 

 browse habitually on seaweed growing upon rocky shores, as the regular 

 Echini do. This is probably due to their comparative helplessness and inca- 

 pacity of locomotion, which they share with the Petalosticha. It is difficult 



