The Crayfish. 43 



the shell above, the interior of the body is seen to be almost 

 entirely filled with the various viscera. Chinks of an irregular 

 form will be visible between the white masses of the muscles ; but 

 there is no spacious cavity like that of the frog or earthworm. 

 In the crayfish, in fact, the body cavity is greatly reduced, 

 and what there is in the way of irregular spaces is not coelom, 

 but is made up of blood spaces in communication with the 

 heart. There are, however, perhaps traces of the coelom in 

 two places : in the interior of the generative gland and at the 

 extremity of the renal organ. But these matters will be 

 entered into when the organs in question are described. The 

 muscular system of the crayfish is enormously developed. 

 The muscles form masses which pass from segment to segment 

 and between the various joints of the appendages. Their 

 detailed description, however, would occupy more space than 

 can be allowed here. When the body is opened from above, 

 w the first organ that comes into view, commencing from above, 

 is the heart. This is a thick-walled sac of somewhat hexagonal 

 form. It lies in a thin-walled sac, which is often — but 

 erroneously — called the pericardium. The strong muscular 

 wall of the heart is perforated by six apertures, the ostia. Of 

 these two are dorsal, two lateral, and two more ventral. They 

 permit a free entry of blood from the enveloping sac into the 

 heart ; but the valves with which they are provided internally 

 prevent the egress of blood from the heart into the investing 

 sac. This so-called pericardium cannot be compared with the 

 pericardium of the anodon of the frog, for several reasons. 

 In the latter animals the pericardium is a portion (in anodon 

 practically the whole) of the coelom, which has no relation to 

 the vascular system. In Astacus the so-called pericardium is 

 to be regarded simply as a number of fused veins whose 

 originally separate orifices into the heart are the existing ostia. 

 It is better to term it the " auricle." 



It is an auricle opening into the ventricle which happens 

 incidentally to envelop it. The pericardium of the other animals 

 mentioned is not connected by openings with the heart which 

 lies in it. The accompanying diagrams may serve to explain the 

 way in which the peculiar auricle of the crayfish's heart arose. 



