434 THE ANATOMY OF INVERTEBRATED ANIMALS. 



but the cerebral ganglia. In other words, the intestine is 

 bent in the opposite direction to that which it takes in the 

 Cephalopod, or has a haemal flexure. 1 



The haemal flexure of the intestine is very characteristic 

 of the JEtranchiof/asteropocla, and is completed at an early 

 stage of their development. 



In such a slightly-modified Odontophoran as Chiton, the 

 heart presents its normal position in the posterior region of 

 the haemal face of the body, and has its aortic end turned for- 

 ward. Although the branchiae are situated at the sides of 

 the body, the blood which passes through them must take a 

 backward course to reach the heart; and thus the branchiae 

 may be said to be virtually behind the heart, and the animal 

 is truly opisthobranchiate. It appears to be otherwise with 

 such a Gasteropod as Buccinum, in which the gills lie actual- 

 ly in front of the heart, and the animal is therefore said to 

 be prosobranchiate. It must be recollected, however, that, 

 strictly speaking, no Odontophoran is other than opistho- 

 branchiate. The anus represents the morphological hinder 

 end of the body ; and the auricle of the heart, into which the 

 current of blood from the branchiae passes, is never, morpho- 

 logically, posterior to the branchiae. 



This is perfectly obvious in the Cephalopoda. In the 

 position which the animal frequently assumes and in which it 

 is ordinarily represented, the gills are in front of the heart. 

 But if the Mollusk is placed in its morphologically correct 

 position with the oral face of the arms downward, it will at 

 once be seen that what is commonly called the ventral face 

 of the animal is the posterior half of its haamal face, and that 

 the heart lies, morphologically, anterior to the branchiae. 



In such Branchiogasteropods as are prosobranchiate, the 

 gills come to lie in front of the heart in consequence of their 

 having followed the twisted intestine forward and to the 

 hgema] side of the body. 



THE PxEROPODA. 2 In this group of small pelagic animals 

 there is no distinct head, the eyes and the ordinary tentacles 

 remaining rudimentary. Auditory sacs are attached to the 

 pedal ganglia. Sometimes (JPneumodermon) two eversible 



1 Huxley, " On the Morphology of the Cephalous Mollusca." (" Phil. 

 Trans.," 1852.) 



2 See Rang and Souleyet, " Histoire naturelle des Mollusques Pteropodes ; " 

 and Gegenbaur, " Untersuchungen iiber die Pteropoden und Heteropoden," 

 1855. 



