ECHINODERMATA. 205 



protruded and retracted by certain muscles, which 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. These processes are short in Clypeaster, and obsolete in 

 Spatangus. For the particular description of these masticatory muscles, 

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

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

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

 versi, — I must refer to the Legons cC Anatomie C&mparee of Cuvier 

 (CLXVIL), the Article by Professor Sharpey (CLX. p. 38, Jig. 17.), 

 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, Jig. 98.), conducts to the gastric or C£ecal portion of the 

 intestine (d) ; and that canal twice performs the circuit of the abdo- 

 minal 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 swallowed with the sand, to a large vessel 

 or vein, which accompanies the intestine from the anus to the mouth, 

 where it terminates in the vascular circle around the oesophagus, 

 from which the arteries are given off for the supply of the whole 

 body. 



In the Spatangus a moderately long caecum is developed from the 

 first coil of the intestine. In the Clypeaster there are several shorter 

 lateral caeca ; and both these and the convolutions of the heavily-laden 

 alimentary canal are supported by peritoneal lamellje from the inner 

 surface of the shell. 



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 well as internal organs of respiration in the 

 Echinoids : the former are the short, pyramidal, branched or pinnate 

 hollow processes, attached by pairs to the oral extremities of the inter- 

 ambulacral areae, and consequently ten in number : their outer surface 

 is highly vibratile. The internal branchias are the transversely ex- 

 tended hollow bases of the tube-feet, which are covered with so rich a 

 network of vessels that Valentin compares them with the lungs of 



