ALIMENTARY SYSTEM 595 



The colon is important chiefly in land forms, where it absorbs 

 water. In fishes it is usually only a short passage for storing 

 faeces, a function carried out in mammals by the rectum. This 

 last is homologous with the part of the gut which in most verte- 

 brates, except the teleosts, receives the genital and urinary ducts, 

 and is called the cloaca. This also is a storage place, from which 

 faeces are voided at intervals. 



Blind diverticula of the gut, or caeca, occur at various positions 

 in most vertebrates, but their distribution is often irregular 

 even within a class. Their function is usually that of allowing 

 increased bacterial digestion or absorption of water, and their 

 commonest position is at the junction of small intestine and colon. 



CIRCULATORY SYSTEM 



All vertebrates are large enough and active enough to need 

 a circulatory system, and in all of them it is based on the same 

 plan : a ventral part in which the blood flows forwards, and a 

 dorsal in which it flows backwards, the two being connected by 

 capillaries. At some point there are interpolated respiratory 

 organs — lungs, gills or skin — and in the ventral portion is a pump- 

 ing organ, the heart. This makes a new division in the system, 

 so that vessels which take blood from the heart to capillaries 

 are called arteries, and those which return it are called veins. 



The original plan of the vertebrate circulatory system seems 

 to have been for the main vessels to be paired, but there is a 

 general tendency for their fusion to form single median tubes. 

 The first ventral vessels to appear in ontogeny are a pair of 

 vitellines, which fuse to form a single subintestinal vessel. In 

 the anterior region, either just before or just after fusion, 

 swelling and specialisation begin and the heart is formed. 

 Primitively it has four chambers, which are, from behind forwards, 

 a thin-walled sinus venosus, a shghtly thicker-walled auricle 

 (or atrium), a strongly muscular ventricle, and a thick-walled 

 and many-valved conus arteriosus. The dogfish shows this 

 condition except that the heart has bent into an S-shape, so 

 that the sinus venosus comes to he above, and the auricle above 

 and slightly in front of, the ventricle (Fig. 246). All fishes are 



somewhat similar. 



With the development of lungs, oxygenated blood is returned 

 to the heart by pulmonary veins before it is sent round the body. 



