XXXIV 



Introduction. 



increased by the addition to it of coecal diverticula^ which may 

 be very numerous, and arranged in whorls in Fishes^ but are never 

 so numerous, nor so arranged in higher Vertebrata. Oral salivary 

 glands are often wanting in aquatic Vertebrata^ the pancreatic are 

 less frequently, and the hepatic is never absent. Microscopic ab- 

 sorptive, as well as secreting glands, exist in great abundance in 

 the walls of the digestive tube. 



With the exception of AmpJdoxus, all Vertebrata possess a 

 lymphatic as well as a blood-vascular system; and the ultimate 

 ramifications of the former of these systems have been recently 

 shown to be continuous with the perivisceral or pleuro-peritoneal 

 cavities. The blood-vascular system, on the other hand, never 

 communicates either with the perivisceral or with the inter- 

 muscular spaces, and the efferent arteries are all but invariably 

 connected with the efferent veins by means of capillaries, with 

 walls distinct from the tissues they pass through. With the 

 exception of Amj^hioxus, in which animal we find all the main 

 vascular trunks endowed with contractility, all Vertebrata possess 

 a heart of saccular shape, consisting even when most simple of two 

 chambers, one of which receives the blood returned by the veins 

 from the system at large, whilst the other propels that blood into 

 the aerating organs. Thus the heart of the Vertebrata is a respi- 

 ratory, whilst that of the Invertebrate animal is a systemic heart. 

 The formation of retia mirabilia, by the breaking up of arteries 

 into plexuses, in the interstices of which no great amount of 

 interstitial matter is deposited, appears to be a peculiarly verte- 

 brate arrangement ; as is also the so-called ' portal system,^ which 

 appears to be formed by the development of retia miralilia, in the 

 course of the veins returning from the chylopoietic viscera, and the 

 intercalation of the elements of the hepatic glands in the interstices 

 of the plexuses thus formed. 



The blood of all Vertebrata, except Amphioxus, is red ; the 

 colouring matter being contained in corpuscles, which appear to 

 be developed from the white corpuscles which are always found in 

 company with them. The spleen and thymus glands are connected 

 like the lymphatic glands, and many other but smaller bodies of 

 somewhat similar histological character, found in the substance both 

 of mucous and serous membranes, with the process of haemato- 

 poiesis ; and appear to be structures peculiar to Vertebrata. 



