120 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



singular resemblance to the foot of Molluscs. (See 

 page 78.) Though the epistome is probably a guard 

 for the mouth in most of the Bryozoa that possess it, 

 it is undoubtedly an organ of locomotion in the re- 

 markable genus Rhabdopleura. The enteric tract is 

 folded on itself, so that the anus is always near the 

 anterior end ; it is, indeed, placed either within 

 (Endoprocta) or without the circlet of tentacles 

 (Ectoprocta). 



Among the Echinodermata, where the mouth 

 is ordinarily placed in the centre of the disc, we 

 find that there are either no masticatory organs, 

 as in the Crinoids, or that the hard skeletal pieces 

 are specially modified in the region of the mouth 

 to form the so-called " odontophore " of starfishes, or 

 the various kinds of mouth papillae which are found 

 in the Ophiuroids. These are ordinarily said to have 

 a masticatory function, but their small size and feeble 

 development justifies us rather in looking upon them 

 as mere filters. In the regular Echinoids, or those in 

 which the spherical form of the body is retained 

 throughout life, a very elaborate system of penta- 

 merally arranged parts is developed, the appearance of 

 which, en masse, has given rise to the popular term of 

 "Aristotle's Lantern." Each fifth part of this lantern 

 consists of a hard tooth, bevelled at the free edge like 

 that of a rabbit or a rat, so as to keep constantly a 

 sharp free edge ; this is supported in a framework, 

 and connected by muscles with an arched piece 

 (auricle) developed on the interior of the test. In 

 Echinanthus and its allies this " dental pyramid " is 

 less complex, and in the Spatangoids it has disap- 

 peared altogether ; so that these last are reduced to 

 living on such organic material as is to be found in the 

 sand, which they scoop up by the aid of their spout- 

 shaped mouths. Holothurians, likewise, are without 

 any special dentary organs, though the walls of their 



