﻿2 BRITISH FOSSIL CEPHALOPODA. 



they have the closest relations. In order to understand these relations we must 

 suppose the animals to be placed in the same positions, the first part of the 

 alimentary canal being taken as the axis. If this be placed horizontally, so that the 

 odontophore is on the lower side of the mouth, we may speak of the under side of 

 the animal as ventral and of the upper side as dorsal. 



There seem to be no valid grounds for objecting to these terms, so long as they 

 are used to represent the same parts in an invertebrate animal ; nevertheless the 

 terms " haemal " and " neural" have been proposed as substitutes by Huxley. In the 

 position indicated, the first, or cerebral, pair of ganglia would be on the upper, or 

 dorsal, side ; while the second, or pedal, pair would lie below, or ventrally. Hence, 

 if we required any substitute, a less misleading one would be " cerebral " and 

 " pedal," 1 since the neural organs lie on both sides, and the hsemal organs on either 

 side of the axis, indifferently. When molluscs of these several classes are placed in 

 this way, they may all 2 be represented, at one stage or other of their development, 

 by a body with a central tube, open at one end, and having at the other the mantle 

 covering, which prevents the tube from opening, and deflects it either dorsally or 

 ventrally, so that its opening is reversed. On the upper side of the alimentary tube 

 are found the organs of sense, on the lower side, usually, the organs of locomotion ; 

 while the organs of circulation follow more or less the flexure of the intestine. 



The first great distinction of the Cephalopoda is the rudimentary condition, or 

 even absence, of that conspicuous organ of the lower mollusca — the foot ; and the 

 development in its place of locomotive organs of completely different character. 

 The foot, as it is seen in all the classes that possess it, and as it is traced from its 

 earliest commencement in the embryo, is invariably a median outgrowth from the 

 ventral side of the body, which may or may not be divided into parts which succeed 

 each other in a longitudinal direction. It shows little or no tendency to spread in 

 a lateral direction, or to subdivide into symmetrical halves. It is often separated 

 from the dorsal parts of the body by a lateral outgrowth called the epipodium, which 

 is parallel to the foot and lies between it and the organs of sense. Such a median 

 " foot " is scarcely to be recognised in any Cephalopod. Within the funnel of some, 

 as the Sepia and Nautilus, a small tongue-shaped valve is found in a corresponding 

 position, which alone can represent it. In the Lamellibranchs and Gastropods the 

 foot is well developed, while it is almost rudimentary in the Pteropods, whose 

 " wings " are formed by the epipodia. 



The organs known as the " arms " have been considered by some to represent the 

 foot ; but they can only be so considered by a violent distortion of the animal from 



1 See my paper on the " Homologies of the Cephalopoda ;" Ann. and Mag. Nat. Hist., 1879. 



2 See Leydig, " Ueber Paludina vivipara," Zeitsch. f. Wiss. Zo'ol., vol. ii. Gegenbaur, ' Unters. 

 iiber Pteropoden und Heteropoden.' Fol, " Sur le developpement des Heteropoden," &c. ; Archives de 

 Zool. vol. v. Ray Lankester, " Developmental History of Mollusca," Phil. Trans. 1875. 



