306 Mr. J. F. Blake on the 



and, the arms of the cephalopod being taken as homologous to 

 the foot, the intestine is made to begin in a vertical direction, 

 while in all other mollusks (except Pteropods) it is made to 

 commence in, and have generally, a horizontal direction. It 

 would seem, to say the least, more natural that the position 

 of the intestine and the nervous centres should be constant, 

 rather than that the whole animal should be displaced for the 

 sake of the, ex hypothese, greatly modified foot. Prof. Owen, 

 in a recent paper, calling the side on which the cerebral 

 ganglia are placed in Cephalopods dorsal, and the opposite 

 side ventral, states that this is assented to by every malaco- 

 logist. It may be wrong for all that : the foot may be 

 always horizontal ; the animal may grow vertically instead of 

 horizontally ; but what is the proof? Prof. Huxley states it 

 as follows : — " Whether we have to do with a cephalopod or 

 with an ordinary mollusk, the first step in the development is 

 the separation of the blastoderm into a central elevation, the 

 mantle, and certain lateral portions. Now these portions 

 become in the Gastropoda the head and foot ; in the Cepha- 

 lopoda the head and arms. It follows, therefore, that the 

 arms of a cephalopod are homologous with the foot of a gas- 

 tropod." 



Now, at the earliest stage at which such organs are recog- 

 nizable, we have, for example, in Paludina vivipara (Leydig, 

 Zeitsch. fiir wiss. Zool. ii. 1850, p. 127, &c.) the alimentary 

 canal in a straight line, a median outgrowth on one side (the 

 foot), and on the other a raised ciliated circle (the velum) ; 

 subsequently the growth of the shell and mantle near the 

 anal end, but slightly on the foot side, displaces the anus 

 forward by taking its place at the end of the intestinal axis. 

 Subsequently the foot grows out behind and before, so that its 

 main axis becomes parallel to the alimentary canal. If now 

 we place the mantle at the top and the mouth at the bottom, 

 we may call the velum and tentacles on one side, and the foot 

 on the other, lateral outgrowths ; but the alimentary canal 

 will run, as in a cephalopod, straight into the mantle-cavity, 

 which direction remains (as far as the stomach) unchanged 

 during development, while the foot does change its position by 

 its fore and hind outgrowths. In the development of the 

 Cephalopoda the partial segmentation of the ovum and the 

 possession of a large yelk or nutritive vitellus displace the 

 mouth, which should arise on the underside of the mantle 

 elevation, and causes it to appear near the circumference of 

 the blastoderm, the anus appearing later at the opposite end 

 of the diameter ; hut there is never a straight canal between 

 them ; their cavities both grow into the hollow of the mantle- 



