636 ZOOLOGY. 
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 Jacune in the spaces between the muscles and 
viscera. 
We have seen that in vertebrates the “‘ aortic heart ” of the 
lancelet or Amphioxus is simply a pulsating tube, and there 
are portions of other vessels which are pulsatile, so that 
there 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, in 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. Jt 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 aset of vessels conveying the 
nutrient fluid or chyle which filters through the walls of 
the digestive canal to the blood-vessels ; these are the dym- 
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. 
