2 COMPARATIVE ANATOMY. 



radiate from the outer surface of the stomach to the inner wall 

 of the outer integument, and thus divide the space between the 

 two the body cavity into a number of wedge-shaped com- 

 partments. Some of these ' mesenteries ' fail by greater or less 

 intervals to reach the outer surface of the stomach, and are 

 therefore called 'secondary' or 'tertiary' mesenteries, while 

 those that are attached to that organ and to the outer in- 

 tegument are called 'primary.' Between them are seen some 

 of the generative glands. The cord-like ' craspeda ' are also to 

 be seen, attached to the inner edges of the mesenteries. 

 Bolleston, p. 158. 



SUB-KINGDOM ECHINODERMATA. 



4. A Common Cross-Fish (Asterias rubens). 



The animal consists of a discoid body, produced horizontally 

 into five radial prolongations. The ventral surface of the disk 

 is depressed into a cavity having a pentagonal outline, and 

 which is further continuous with the mesial lanceolate grooves 

 extending along the ventral surfaces of each ray. The dorsal 

 surfaces of the disk and rays, on the other hand, are convex and 

 not grooved. In the middle of the ventral surface of the disk is 

 the mouth: and on the corresponding part of the dorsal surface, 

 the anus. Protruding from the radial grooves or avenues are the 

 csecal tube-feet : arranged in four rows, two on either side of a 

 line running down the centre of each ray. There are two sets 

 of spines bounding each avenue, 'one of which is placed in- 

 ternally and consists of two rows of long and slender spines: 

 while the more externally placed is made up of three rows of 

 shorter and blunter spines.' Externally to the latter is a third 

 series of spines, marking the line of junction of the dorsal and 

 lateral surfaces. Furthermore the dorsal surface of both rays 

 and disk is studded with variously shaped spines, some short 

 and obtuse, while others carry a small bunch of calcified setae 

 at their free extremities. None of the spines are moveable. 

 On the extreme edge of the dorsal surface of the disk, and in 

 one of the inter-radial spaces, there is a porous elevation the 

 madreporic tubercle through the medium of which, and of the 

 'sand canal' with which it is in relation internally, the sea- 



