122 LECTURE X. 



triangular pyramid (h), two sides of which are in close apposition 

 with opposite sides of the adjoining pyramids, and are transversely 

 grooved like a file, so as to operate upon the alimentary matters which 

 have been divided by the incisor plates, and which are thus minutely 

 comminuted before they pass into the membranous oesophagus. 



The secretion of some simple salivary follicles assists in completing 

 the mastication of the food. These singular representatives of molar 

 and incisor teeth are moved upon each other ; and the entire pyra- 

 midal mass, which has been called Aristotle's lantern^ can be pro- 

 truded and retracted by certain muscles, w^hich have their fixed points 

 of attachment in five calcareous ridges and arches which project from 

 the inner surface of the plates near the margin of the oral vacancy of 

 the shell. For the particular description of these masticatory muscles, 

 which are classed under the following heads, 1. Musculi interarcuales, 

 s. comminutores ciborum, 2. Musculi arcuales, s. dilatores orificii den- 

 tium, 3. Musculi interpyramidales (sphincter oris), 4. Musculi trans- 

 versi, — I must refer to the Lemons d! Anatomie Comparee of Cuvier, 

 and the monograph of Professor Valentin, already cited. 



The pharynx occupies the cavity of the lantern, and is divided by five 

 longitudinal folds, most prominent at their commencement ; the small 

 salivary caeca are placed close to its continuation with the oesophagus, 

 from which it is separated by a marked constriction. A slender oesopha- 

 gus (c) conducts to the gastric or caecal portion of the intestine (^); and 

 that canal twice performs the circuit of the abdominal cavity before 

 its final termination. The vent, its membrane, and the anal plates have 

 appropriate muscles for constriction and dilatation. The intestine is 

 generally found more or less loaded with fine sand ; its surface and 

 that of its mesentery is covered with a rich vascular network, which 

 conveys the nutrient fluid eliminated from the organic particles swal- 

 lowed with the sand, to a large vessel or vein, which accompanies the 

 intestine from the anus to the mouth, where it terminates in the vas- 

 cular circle around the oesophagus, from which the arteries are given 

 off for the supply of the whole body. 



The sea water is admitted into the peritoneal cavity ; and its con- 

 stant renovation over the surface of the vascular membranes of the 

 Echinus, is provided for by the same mechanism of vibratile cilia as 

 in the Asterias. 



There are external as w^ell as internal organs of respiration : the 

 former are the short, pyramidal, branched or pinnate hollow pro- 

 cesses, attached by pairs to the oral extremities of the interambulacral 

 areae, and consequently ten in number. Their outer surface is highly 

 vibratile. 



The internal branchiae arc the transversely extended hollow bases of 



