20 W. K. BROOKS ON THE 



locomotor oi'gan, the foot of the Cephalopod has been suppressed by the great development 

 of a food-yolk at the j)oint where it should be found. 



The arms of the Squid luake their appearance as little protrusions, fig. 19, a, a, a, arranged 

 in pairs around the neck or constriction which separates the external yolk-sac from the 

 body proper. As they are, at first, ventral to the mouth, and as a true velum is present, 

 they cannot be regarded as a velum, and as they are paired instead of median, they are 

 not homologous with the median unpaired foot. They are paired outgrowths from the foot 

 region, and may perhaps be regarded as the equivalents of the cephaloconi of Clio, but 

 there does not seem to be any evidence that they have been produced by the modification 

 of any part of the body of a typical Gasteropod, and they are undoubtedly structures which 

 have been acquired by the Pteropods and Cephalopods, after these diverged from the 

 common ancestral form which united them to the Gasteropod stem. 



The siphon originates as two pairs of folds, s and s', of the integument of the lateral 

 walls of the body, and if we regard these four folds as homologous with the epipodial 

 folds of a Gasteropod, the arms must be regarded as independently acquired structures. 



If we regard the arms as modifications of the epipodial folds we must consider the four 

 siphon folds as independeutly acquired structures, and as we have nothing whatever to 

 furnish us with a test, nothing seems to be gained by the uncertain homology of either 

 the arms or the siphon, with any part of the body of a typical Gasteropod. 



It seems certain that the common ancestor of the Gasteropods and Cephalopods must 

 have been an unspecialized form, rather than a highly complex architype, and if this is the 

 case we cannot expect any valuable results to follow from the attempt to compare any 

 part of the body of a Cephalopod with structures which, like the epipodial folds, are not 

 common to the Gasteropoda, but somewhat exceptional ; which, when they are found at 

 all, as in Aplysia, are not rudimentary but functional ; making their appearance very late 

 in the history of the individual instead of early, and presenting every indication of recent 

 acquisition. 



While we owe a great debt to Huxley's paper on the Morphology of the Cephalous 

 MoUusca, for the demonstration of the general relations between the Cephalopod body 

 and that of a Gasteropod, I think that confusion has resulted from his attempt to 

 carry the homology into the details of Cephalopod structure. 



The growth of opinion upon the homology of the siphon and arms may be stated 

 briefly as follows : 



Huxley regarded the arms as the true foot, and the siphon as the epipodial folds. 



Grenacher shows that as the foot is an unpaired structure, it cannot be homologous 

 with the arms, and he follows Lov^n (Beitrage zur Kenntniss der Entwicklung der Mollusca 

 Acephala Lamellibranchiata ; reprint of 1879, p. 33), in regarding these as a modified 

 velum. The foot he believes to be wanting, and the siphon he regards as the epipodium. 



Von Jhering opposes Grenadier's view that the arms are homologous with the velum, 

 and points out that the cephaloconi of CHo, which are undoubtedly homologous with the 

 arms, are certainly not homologous with the velum, since an embryonic velum appears in 

 the young Pteropod, and then disappears without forming any part of the adult body. He 

 also calls attention to the fact that in the Gasteropods, even when the velum is cut up into 

 tentacles, these do not persist or become converted into any part of the adult body. He 



