492 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



its tissues are the representatives of a blood-vascular system a condi- 

 tion closely analogous to what occurs as the first indication of a circula- 

 tion in plants. In annelida, as is also the case in the rotifea, we find a 

 perivisceral cavity lying between the splanclmopleure and the somato- 

 pleure, communicating with the segmental organs, as the water-vascular 

 system. In the former group there is also to be found a system of canals 

 (the pseudo-haemal system), in some instances communicating with the 

 perivisceral cavity, with contractile and often ciliated walls, and con- 

 taining a clear, sometimes corpusculated fluid, which may be either red 

 or green from the presence of a substance which resembles haemoglobin 

 and which is evidently of a respiratory value. These canals always com- 

 municate at some point b}^ a tubular stem with the exterior. In the 

 lowest forms of the arthropoda the same general conditions noted in 

 the turbellaria are to be found, viz., a perivisceral cavity and an inter- 



si. A 



IAA 



FIG. 180. DIAGRAMS TO SHOW THE ARRANGEMENT OP THE GREAT BLOOD- 

 VESSELS IN WORMS AND LOWER CRUSTACEANS. (Jeffrey Bell.} 



A, earthworm : D. dorsal vessel. B, crayfish: A A, anterior aorta ; PA, posterior aorta ; H, heart; 

 AT and HP, transverse vessels which supply the anterior regions of the body and the viscera; ST.A, 

 sternal artery ; SI.A and IAA, abdominal artery. 



stitial fluid, in which, however, colorless cells may be detected. In the 

 lower Crustacea and in many insecta we find a single elongated, some- 

 times segmented, contractile vessel, the dorsal vessel, provided with 

 lateral valvular openings by which the blood enters from an inclosing 

 venous space or sinus (Fig.180). In the anodon (Fig. 181, C) the spaces 

 or sinuses are much more developed, and no traces of a ventral vessel 

 are now to be seen ; the dorsal is, however, shown by the heart, with its 

 anterior and posterior aortae, while the terminal parts of the transverse 

 vessels become enlarged to form the auricles of the heart. In the higher 

 Crustacea, as in the lobster, there is a single, well-developed, muscular, 

 systemic dorsal heart surrounded by a venous sinus and giving off a 

 number of arteries, which pass into capillaries ; but the venous system 

 still remains more or less lacunar. In the mollusca, also, the same gradual 



