ZOOLOGY. 



361 



The manifestation of such different types of dentition by forms hav- 

 ing the same kind of shell will render a re-examination of the other 

 parts of the animal necessary before it can be definitely accepted whether 

 they are distantly related, as the current views respecting the value ot 

 dentition would imply, or whether they are closely connected, as the hke- 

 ness of their shells would suggest. In this connection we may refer to 

 the similarity of the shells of the Mitridie and Turritidie. While it is 

 not safe to assume that the animals so distinguished are closely related 

 on account of the similarity of their shells, there is enough at least to 

 challenge doubt, which can only be settled by further investigations. 

 (See Crosse & Fischer in Journ. Conch yliologie {3,) vol. xx, p. 375, 

 1880; and Justus Carriers (Die Gattung Pseudomarginella v. 

 Maltzan), in Zool. Anz., vol. iii, pp. 037-G41, 27 Dec, 1880.) 



CBPHALOPODS. 



THE ARMS AND SIPHON OF CEPIIALOrODS. 



A number of attempts have been nuule to homologize the arms and 

 siphon of the cephalopods with the structures of the gastropods, but 

 the identifications have not been entirely satisfactory. Professor Loven, 

 as early as 1848, regarded the cephalopod arms as a persistent 

 velum. Professor Huxley considered that they represented the gastro- 

 pod foot, and the siphon was regarded as answering to the epipodial folds 

 of the embryonic gastropod. Dr. Grenacher, however, showed that the 

 foot of the gastropod is an unpaired structure, and therefore could not be 

 homologous with the arms, which develop hi pairs, and, like Loven, he 

 considered that the arms represent a modified velum ; the foot was 

 deemed to be entirely wanting, while the siphon was regarded as the 

 representative of the epipodium. Dr. Von Ihering contended that the 

 arms are tentacular appendages to the body, and that they have no 

 equivalent in the gastropods. The innervation of the siphon from the 

 pedal ganglion led him to identify the siphon as the foot, and he urged 

 that the valve of the siphon is a true foot or protopodium, and the two 

 lateral folds pteropodia. 



In 1870 Mr. J. I. Blake entered upon a comparison of the cephalopod 



and gastropod structures with reference to their relation to the nervous 



ganglia and the- flexure of the intestine.* Premising that the ciliated 



bands constituting the velum had been shown to be traceable "from one 



class to another— sometimes in the larva only, and sometimes as an 



adult organ "—he concludes as follows : " In the primitive condition they 



formed a circle around the oesophagus, and as often as not are thrown 



out into long processes ; with a change in the direction of the intestine 



their uniformity is broken and part dies away, while the other part is 



left, forming a circle surrounding, not the cesophagus, but a portion of 



the body on one side of it, the foot being on the other. Since, then, in the 



gastropoda the intestine tur ns to the cerebral side, we have the "velum" 



* Blake (J. F.). Ou the Homologies of the Cephalopoda. Ann. Mag. iVaf. Hist. 



(5,) vol. iv, pp. 303-312. 



