362 ZOOLOGY. 



formed on that side; whereas in the cephalopoda, the flexure being on 

 the oi^posite side, we have what we may call the ant I- velum on the pedal 

 side. Thus the arms are homologous to the opposite portion of the 

 architroch to that which forms a velum, and merely afibrd another in- 

 stance iu which these primitive formations are retained as functional 

 organs. Moreover, from within the circle of the embryonic velum rise 

 up in souie gastropods two loug retractile tentacles in like manner 

 from within the later-formed circle of the anti-velum rise up the two 

 retractile tentacles of the decapods." The funnel is conceded to be 

 homologous with the cpipodium, and the valve in the funnel it is sug- 

 gested "may represent" the foot, although this "must be doubtfid." 



Dr. Brooks has also treated of the same questions, and studied 

 the development of the common squid {Loligo Fealli).* lie recalls 

 that the molluscan foot is an unpaired structure on the ventral surfa(!e 

 of the body between the mouth and the anus ; that in the embryo a large 

 sinus space separates the integument of the foot from the endoderm and 

 its derivatives ; and that this space contains blood corpuscles ; that the 

 embryonic foot, in fact, is a circulatory organ. The yolk sack of the 

 cephaloi)od is separated from the yolk by a corresponding sinus space, 

 and as the integument is rhythmically the duid whicli fills this space is 

 kept in constant motion. Physiologically then, as well as in its posi- 

 tion, the yolk sack of the squid resembles the foot of the gastropod, and 

 consequently it is contended that we find in it the homologue of the foot. 

 He concludes that as a locomotor organ the foot of the cephalopod has 

 been suppressed by the great development of a food-yolk at the point 

 where it should have been found." As to the siphon, this originates as 

 two pairs of folds of the integument of the lateral walls of the body, ^nd 

 if we regard these four folds as homologous with the epipodial folds of 

 a gastropod, the arms must be regarded as independently acquired 

 structures. It is finally inferred that neither the exact equivalent of the 

 arms nor of the siphon is found in the gastropods, but that these parts 

 of the cephalopods have originated from a very generalized condition 

 'and become developed as specialized organs peculiar to the class; con- 

 versely, the foot of the gastropods is represented by no specialized struct- 

 ure iu the cephalopods. 



COMPARATIVE STRUCTUEES OF THE TETRABRANCHIATE AND 

 DIBRANCHIATE CEniALOPODS. 



As a part of his studies on the "homologies of the cephalopoda," ^Iv. 

 Blake has examined into the structures which seem to correspond in the 

 nautiloids and cuttle-fishes. The tetrabrauchiates are undeniably a 

 more primitive and generalized type than thedibranchiates, and in their 

 adult condition exhibit characteristics which are transitory in the other. 

 Thus (1) in the nautilus the funnel is divided, and so it is in the embryo 



* Brooks (W. Iv.). lioinology of the Cephalopod >Sipliou aud Arms. Am. Jour. Sci. 

 (3), vol. XX, pp. 288-291 ; also, The Developmeut of the Squid Loligo IValii (Lescreur) 

 Anniv. Mem. Boston Soc. Nat. Hist. 



