ZOOLOGY. obL 



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

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

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

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

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

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

 the similarity of the shells of the Mitridie and Turritida). 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. Conchyliologie (3,) vol. xx, p. 375, 

 1880; and Justus Carriers (Die Gattung Pseudomargiuella v. 

 Maltzan), in Zool. Anz., vol. iii, pp. G37-G41, 27 Bee, 1880.) 



CBPHALOPODS. 



the arms and siphon of cephalopods. 



A number of attempts have been made 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 1818, regarded the (;ephalopod 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. Greuacher, 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. Yon Ihering contended that the 

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

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

 pedal ganglion led hini 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 1879 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 cesophagus, 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 oesophagus, but a portion of 

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

 gastropoda the intestine turns to the cerebral side, we have the " velum " 



* Blake (J. F.). On the Homologies of the Cephalopoda. Ann. Mag. Nat. Hist. 

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



