﻿BRITISH FOSSIL CEPHALOPODA. 3 



its natural position, and a setting aside the guidance of adult relations and early 

 development. These arms are, from their very origin, in pairs ; so though they 

 arise on the ventral side of the embryo, they are lateral and not median outgrowths. 

 Moreover, they are never separated from the mouth and organs of sense by the 

 epipodial line. They change their position with growth, and come ultimately to 

 surround the mouth and to lie between it and the eyes. At no stage therefore of 

 their development do they correspond in any way to the foot ; but they are, on the 

 contrary, the distinguishing organ of the Cephalopoda. Their origin may, however, 

 be somewhat doubtfully traced in connection with other outgrowths of the embryo. 

 They have been by many considered to represent the velum of the embryo Gastropod 

 and Pteropod, which arises at first as a circlet of cilia, but soon develops into lateral 

 lobes. 1 This velum so far agrees with the cephalopod arms, that it is in front of the 

 eyes and behind the tentacles ; but it never comes to surround the mouth, and it 

 commences on the dorsal, and not on the ventral, side of the embryo. But the 

 velum itself is but a particular modification of a more general outgrowth, to the 

 primitive, theoretical form of which the name of " architroch " 2 has been given. 

 This is variously modified from its original form of a circle surrounding the mouth ; 

 and, as traced through the several sub-kingdoms, is seen to be much influenced by 

 the curvature of the intestine, to arrange itself in various ways with respect to the 

 mouth, and to break up into halves, of which only one may be persistent. It may 

 therefore be suggested that, as the velum of Gastropods is the dorsal half of the 

 architroch, which does not surround the mouth, so the arms of the Cephalopod are 

 the ventral half that does surround the mouth, and they may therefore be called an 

 " antivelum." 3 



A second general characteristic of the Cephalopods is the partial segmentation 

 of the ovum, the result of which is that the axis of the embryo is thrown out 

 obliquely, and the mouth, instead of being in the centre of the anterior end, is at 

 one side, and the anus at the other ; while the mantle is central at the posterior 

 end. This distortion rather interferes with the recognition of the true position of 

 the several organs, and it is only after some time that, by the elevation of the embryo 

 from the yelk, the mouth approaches its normal position. 



A third peculiarity of the Cephalopoda, in which however they agree with the 

 Pteropods, is that their intestine is bent towards the ventral, and not towards the 

 dorsal, side of the body. The main organs of circulation have followed in the same 

 direction, and the heart and gills are thus situated on the lower, and not on the 



1 See Grenadier, Zeitschrift fur Wiss. Zool., 1874, who gives an account of the various opinions 

 held. 



2 Eay Lankester, Quart. Journ. Micr. Sc, New Series, vol. xvii. 



3 Eay Lankester, loc. cit., calls the velum a " cephalotroch," and this antivelum would he a 

 " branchiotroch," but the use of this latter term in the present case would be misleading. 



B 2 



