196 



BIOLOGY. 



[BOOK ii. 



diactile, they have even sometimes several. (Figure 18.) There 

 has been an attempt to show the distinct existence in the heart 



of the mollusks of a ventricle and 

 of auricles; but the ventricle is 

 only a dilatation of the dorsal 

 trunk, and the auricles are only 

 dilatations of the transversal vessel 

 debouching into the pretended ven- 

 tricle. In the cephalopods the dorsal 

 vessel curves like a curl, the bran- 

 chial veins debouch thereinto by 

 dilating. The heart sends forth 

 two arteries, the one cephalic, the 

 other abdominal. There is a true 

 network of fine capillaries between 

 the last ramifications of the venous 

 and arterial systems. Most of the 

 cephalopods have cardiacal mus- 

 cular dilatations on the branchial 

 arteries. Spite of the relative per- 

 fection of this apparatus, the cavity 

 of the body constitutes always a 

 huge lacuna full of blood, which 

 bathes the organs direct. This cavi- 

 ty communicates with the vessels j 

 veins debouch direct thereunto. 



The circulatory system attains its 

 completion in the vertebrates, but 

 it is sti11 Vei 7 imperfect in the first 



o jf them, the amphlOXUS. In effect 



in this animal, which Haeckel 

 wishes to regard as a connect- 

 ing point between the mollusks and the vertebrates, there 

 is still no heart, and the impulsion is impressed on the san- 

 guineous liquid by all the larger vessels, which are contractile. 



FIG. 17. 



Schematic figure of the circulatory 

 apparatus of the lobster : o, eyes ; a e, 

 exterior antennae ; a i, interior anten- 

 nas ; b r, branchiae ; c, heart ; p c, 



T 1 J ' 



liver; a p, posterior artery of 

 body ; a, trunk of ventral artery ; a v, 

 anterior \ entral artery ; v, ventral 

 venous sinus ; v "b r, branchial veins. 

 The arrows indicate the direction of 

 the sanguineous currents. 



