i 3 o COMPARATIVE ANATOMY 



with blood. It will be observed that, though the heart is 

 very short in comparison with that of Apus, it may be regarded 

 as a muscular dilatation of the median blood-vessel formed by 

 the ophthalmic artery in front and superior abdominal artery 

 behind; it is, in fact, a specialised contractile portion of a 

 great dorsal longitudinal vessel, such as is represented by the 

 heart of Apus and the dorsal vessel of the earthworm. 



All the arteries enumerated above ramify in the organs to 

 which they are supplied, and their ultimate ramifications are 



ar.b. 



Fig. 30 



The right branchial chamber of the crayfish, [exposed by cutting away the 

 branchiostegite. The podobranchiae are turned downwards, and every 

 alternate arthrobranch is cut short, a-f, podobranchiae borne on the seventh 

 to the twelfth limbs inclusive ; ar.b, arthrobranchiae ; pl.b, functional 

 pleurobranchia on the thirteenth segment ; //. b^ rudimentary pleurobranchiae 

 on segments 10, n, and 12 ; e.g., cervical groove ; mxp. 3, third maxilliped ; 

 R, rostrum'; Scp, scaphognathite. 



so fine as to have received the name of capillaries ; but there 

 is no true capillary system as in vertebrates. Eventually the 

 ramifications open into larger or smaller spaces or blood 

 sinuses lying between the tissues, and the blood is returned 

 by special passages to the heart. One of the most important 

 of these sinuses is the pericardial space, which has the same 



