COMPARATIVE ANIMAL MORPHOLOGY AND PHYSIOLOGY. 315 



coelomic system. The Crayfish, Mussel, and Snail show no dis- 

 tinction between blood and lymph systems. The blood system 

 in each case is lacunar i.e., the ramifications of the blood-vessels 

 terminate in the lacunse everywhere permeating the tissues. The 

 heart of the Crayfish possesses no auricles ; there are two in the 

 Mussel, but only one (a result of asymmetry) in the Snail. All 

 three hearts are systemic, that is to say, they receive oxygenated 

 blood, which they pump to the body at large. Amphioxus ap- 

 parently possesses freely communicating blood and lymph systems. 

 The absence of a heart is made up for by the contractility of the 

 vessels. In the Dogfish, Frog, Pigeon, and Eabbit, the blood 

 system is closed, owing to the presence of the capillaries, which 

 connect the ultimate ramifications of the arteries and veins. The 

 lymph system is largely lacunar, including also certain large 

 spaces, the coelom, &c., but definite trunks are also present, 

 best marked in the Eabbit and worst in the Dogfish. The Frog 

 possesses propulsive lymph-hearts. In the Dogfish, the heart 

 contains impure blood only, which it pumps to the gills for oxy- 

 genation and elimination of C0 2 . The heart in Frog, Pigeon, and 

 Kabbit is both systemic and pulmonary, supplying the body and 

 also pumping impure blood to the lungs (and in the Frog to the 

 skin also) for purification. Only one ventricle is possessed by. 

 the Frog, which involves complicated mechanical contrivances by 

 which the complete mixing of oxygenated and impure blood in 

 the heart is prevented. The Pigeon and Rabbit have two com- 

 pletely separated ventricles. 



3. Katabolism (1) Respiration. This is aided in Amoeba and 

 Vorticella, by the contractile vacuole. In Gregarina, Hydra, 

 Fluke, Tapeworm, Ascaris, Earthworm, and Leech, it is effected 

 by the general surface of the body, beneath which, in the last 

 two cases, there is a rich supply of blood-vessels. The Crayfish 

 and Mussel possess gills (and the latter mantle-lobes as well) for 

 breathing the oxygen dissolved in water, and in Amphioxus and 

 Dogfish the pharynx is modified for the same purpose, while the 

 Snail, Frog, Pigeon, and Rabbit possess lungs, by means of which 

 the oxygen in the air is utilized. The gills of Crayfish and 

 Mussel are specialized outgrowths of the body ; the mantle lobes 

 of Mussel and lung-roof of Snail may be similarly described ; the 

 lungs of Frog, Pigeon, and Rabbit are ventral outgrowths of 

 the gut. 



