1 84 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



their predecessors ; and any variations which have taken place in the 

 latter will perturb the normal disposition of the former. 



Now in the Mollusca, as has been already stated, the neural side of 

 the embryo is the last to be developed, and the nervous system does 

 not make its appearance until the animal has taken its characteristic 

 form. 



Contrast this with the Vertebrata ; in them the nervous system is 

 the first to be developed, and it is, of consequence, the most fixed and 

 unchanging feature in the whole of their organization. 



On the other hand, the separation of the abdomen or post-abdomen 

 from the body is one of the earliest facts in molluscous development, 

 and it has a corresponding influence over their whole organization. 



Tlie Archetypal Vascular System and its Modifications. — It may be 

 questioned whether the " archetypal " heart has a single or a double 

 auricle, but it is certain that in proportion as the symmetry of the 

 branchial apparatus and of the whole body is preserved, we approach 

 to the form of heart with a double auricle. Thus we have a double 

 auricle in Chiton and Haliotis, and a close approach to it in Tethys, 

 Janus, and the EolidcE. 



In the Cephalopoda the contraction of the branchio-cardiac veins 

 has been observed by Milne-Edwards and KoUiker, so that they may 

 be considered to be auricles. This is another curious illustration of 

 the fact, that what is commonl}' considered the most concentrated 

 and highest organization does not occur in the reputed highest forms 

 of Mollusca. 



The heart lies above the intestine, and gives off the aorta anteriorly. 

 This runs forwards through the cephalo-pedal ring with the oesophagus 

 and terminates eventually in the buccal mass. Its main branches may 

 be classed as visceral and pedal. 



It is needless to enter here upon the beautiful discoveries of M. 

 Milne-Edwards, with respect to the incompleteness of the circulation 

 in the Mollusca. The facts I have detailed add ocular proof to his 

 already convincing demonstrations. But it is to be observed, that in 

 this respect, again, the " highest " Cephalopod, Octopus, possesses no 

 ■" higher " organization than the Slug or Snail. ^ 



The consideration of the archetypal vascular system leads naturally 

 to that of the value of the distinction made by M. Milne-Edwards be- 

 tween opisthobranchiate and prosobranchiate Mollusca. If my views be 

 well founded, it is clear that opisthobranchism is the typical condition 



^ The " vena cava superior '' of Cephalopoda answers to the very short trunk formed by 

 ■the union of the two afiferent branchial trunks in Aplysia, &c., which, as receiving the veins 

 of the foot, correspond with the venous circlet at the base of the arms. 



