196 



BIOLOGY. 



[Book ii. 



dia'ctile, they have even sometimes several. (Figui-e 18.) There 

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



of the moUusks 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 ciu-l, the bran- 

 chial veins debouch thereinto by 

 dUating. "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 ; 

 veins debouch direct thereunto. 



The circulatory system attains its 

 completion in the vertebrates, but 

 it is still very imperfect in the first 

 of them, the amphioxus. In efiect 

 in this animal, which Haeckel 

 wishes to regard as a connect- 

 ing point between the moUusks and the vertebrates, there 

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

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



Fig. 17. 



^cliematic figure of tlie circulatory 

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

 exterior anteunse ; a i, interior anten- 

 na ; & r, branchiae ; c, heart ; p c, 

 pericardiiira ; a o, median and int rior 

 artery of the body ; a a artery of tho 

 liver; a 'p, posterior artery of tlie 

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



• anterior ventral artery ; v^ ventral 

 venous sinus ; w & r, branchial veins. 

 The arrows indicate the direction of 

 the sanguineous currents. 



