DEVELOPMENT OF THE SQUID. 19 



When the corresponding surface of the body of the Cephalopod embryo, at the stage 

 shown in plate 2, fig. 10, is carefully examined with the highest power which can be 

 brought to bear upon such a large rounded opaque surface, a well defined line or groove, 

 plate 3, fig. 19, v, will be found to run from the median line out upon the sides of the 

 body, bending up towards the dorsal surface in such a way as to partially encircle the eye- 

 stalks ; and the motion of floating particles shows the presence of cilia along the line. 



Immediately below it at the point where it approaches the median line of the body, the 

 mouth, m, is situated, in the Cephalopod as well as in the Pulmonate, and the fact 

 that the ciliated line bears exactly the same relation to the mouth and to the sensory 

 tentacles in the Cephalopod that it bears in the Pulmonate seems to show beyond doubt 

 that it is the same structure, a rudimentary velum, in both cases. 



The mantle ; the mantle-cavity ; the shell area ; the shell ; the rectum and anus ; the 

 sensory tentacles ; the velum, and the mouth are thus seen to be so similar in position, 

 relations, mode of development and function, in these two Molluscs, as to leave no doubt 

 of their homology. 



When used as a basis for comparison these various features furnish us with a sufiicient 

 number of points of orientation to assure us that the Cephalopod embryo must be placed as 

 it is in plate 3, fig. 19, and in plate 2, figs. 10 and 11, in order to be in a position which is 

 homologous with that of the Gasteropod embryo, shown in fig. 20. 



The views which were advanced upon the morphology of the Cephalopoda', nearly thirty 

 years ago, by Huxley, are therefore essentially correct in outline, although I shall now 

 give my reasons for opposing certain of the homologies advocated by him. 



There are few morphological questions upon which more conflicting views have been 

 expressed than those of the various writers who have discussed the homology of 

 the siphon and arms of the Cephalopod, and the equivalent, in this group, of the Gasteropod 

 foot. 



The history of opinion upon this subject has been treated at length by Grenacher, 

 and more recently by Von Jhering (Vergl. Anat. des Nervensystems und Phylogenie der 

 MoUusken, pp. 269-281), and I may therefore enter at once upon an examination 

 of the morphological aspect of the question without first reviewing its historical side. 



The moUuscan foot, fig. 20, /, is a median unpaired structure, on the ventral surface 

 of the body, between the mouth and the anus. In the Pulmonate, and in many other 

 Gasteropod embryos, a large sinus-space, c, separates the integument of the foot from the 

 endoderm and its' derivatives. This space contains blood corpuscles, and as the integument 

 is rythmically contractile the embryonic foot is a circulatory organ. 



A glance at fig. 19, or fig. 10, will show that the only unpaired structure on the 

 median line of the ventral surface of the body of the Cephalopod embryo is the large 

 external yolk-sac, y, and to this, if anywhere, we must look for the homologue of the 

 Gasteropod foot. 



When the Cephalopod embryo is seen in a profile view, fig. 19, or fio-. 15 the 

 integument, /, of the yolk-sac will be found to be separated from the yolk by a space, c, 

 and as the integument is rythmically contractile, the fluid which fills this space is kept 

 in constant motion. Physiologically then, as well as in its position, the yolk-sac of the 

 Squid resembles the foot of the Gasteropod, and I think we must conclude that, as a 



