304 Mr. J. F. Blake on the 



is their relation in homology, and thereby in their ontogeny, 

 to the other classes of Mollusca; and the second, the relations 

 of the Dibranchiate and Tetrabranchiate orders. These are 

 questions on which our two great anatomists Professors Owen 

 and Huxley have expressed decided opinions, which have not, 

 however, been accepted by all, or perhaps the majority, of 

 foreign naturalists. Constant accumulations also of new facts, 

 especially in relation to the embryology of the Dibranchiates, 

 force on us a reconsideration of the ideas derived solely from 

 older ones, and even may lead us to put a different interpreta- 

 tion on the latter. 



In order to compare the various classes of the Mollusca, we 

 must place them in similar positions as defined by the first 

 part of their alimentary canal and the circumoesophageal 

 ganglia. The primitive form will then have a straight ali- 

 mentary canal, with the cerebral ganglia above, the pedal more 

 or less below, and the heart near the other end, its afferent 

 vessels coming from the direction of the anus, and its effe- 

 rent going towards the head. From this primitive form the 

 rest may be deduced by a bending forwards of the anal end, 

 carrying with it the heart and its branchiae. On the direction 

 of this flexure of the intestine great stress has been laid by 

 Huxley ; but I have not found much notice taken of it by 

 foreign writers. It is obvious that such a flexure may take 

 place in two opposite directions ; and these have been defined 

 by Huxley, in his recent work on the Anatomy of the Inver- 

 tebrated Animals, as follows : — In the first the cerebral 

 ganglia lie within the general angle formed by the intestine ; 

 in the second it is the pedal ganglia which lie within it. 

 Unfortunately these two directions have been called respec- 

 tively the " hsemal " and "neural." Of course the flexure 

 must in every case be neural, as tending to bring the anus 

 nearer to the nervous centres ; and it must generally be also 

 hsemal, for the heart usually accompanies it in its changes. 

 In particular the Cephalopoda are said to have a neural 

 flexure. In these the intestine is bent to the side of the pedal 

 ganglia ; but yet its direction is towards the heart, which 

 lies on the underside. In the Pulmonata the intestine bends 

 to the side of the cerebral ganglia ; and yet its direction is 

 towards the heart, which lies on the upperside. These latter 

 were formerly said to have a neural flexure ; but it is now 

 called hsemal*. Taking this last view, and substituting the 

 terms " cerebral " and " pedal " for hsemal and neural, the 

 distinction between the classes is most marked, the Pteropoda 



* Huxley, 'Morphology ofCeph. Mollusca,' 1853, and 'Introduction to 

 Classified p. 39, with ' Manual of Invert. Animals,' p. -014. 



