636 ZOOLOO Y. 



nearer approach to the vertebrate heart and its vessels than 

 even the crustaceans and insects, for the ventricle and one or 

 two auricles, with the complicated arterial and venous sys- 

 tem of vessels of the clam, snail, and cuttle-fish, truly fore- 

 shadow the genuine heart and systemic and pulmonary cir- 

 culation of the vertebrates. The mollusks, and king crab, 

 and the lobster present some approach to the capillaries of 

 vertebrates. The circulation in certain worms, from Ne- 

 mertes upward, may be said to be closed, the vessels being- 

 continuous ; but they are not so in insects, where true veins 

 are not to be found, the blood returning to the heart in 

 channels or lacunca in the spaces between the muscles and 

 viscera. 



We have seen that in vertebrates the " aortic heart " of the 

 lancelet or AmpJiioxns is simply a pulsating tube, and there 

 are portions of other vessels which are pulsatile, so that 

 tlicre is, as in some worms, a system of "hearts." A gen- 

 uine heart, consisting of an auricle and a ventricle only, first, 

 appears in the lamprey. This condition of things survives, 

 in fishes, with the exception of those forms, such as the lung- 

 fish (Dipnoans), whose heart anticipates in structure that of 

 the amphibians and reptiles, m which a second auricle ap- 

 pears. Again, certain reptiles, such as the crocodiles, antici- 

 pate the birds and mammals in having two ventricles i.e., 

 a four-chambered heart. It should be borne in mind that 

 in early life the heart of all skulled vertebrates (Craniota) 

 is a simple tube, and as Gegenbaur states, " as it gradually 

 gets longer than the space set apart for it, it is arranged in 

 an S-shaped loop, and so takes on the form which the heart 

 has later on." Owing to this change of form, it is divided 

 into two parts, the auricle and ventricle. 



A striking feature first encountered in the craniate ver- 

 tebrates is the presence of a set of vessels conveying the- 

 nutrient fluid or chyle which filters through the walls of 

 the digestive canal to the- blood-vessels ; these are the lym- 

 phatics. In the lancelet, as well as in the invertebrate ani- 

 mals, such vessels do not occur, but the chyle oozes through 

 the stomach-walls and directly mixes with the blood. 



