362 ZOOLOGY. 



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

 the opposite side, we have what we may call the anti-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 afford another in- 

 stance in which these primitive formations are retained as functional 

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

 np in some gastropods two long 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 epipodium, and the valve in the funnel it is sug- 

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



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

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

 that the molluscan foot is an unpaired structure on the ventral surface 

 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, iu fact, is a circulatory organ. The yolk sack of the 

 cephalopod is sej^arated from the yolk by a corresponding sinus space, 

 and as the integument is rhythmically the fluid which 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, and 

 if we regard these four folds as homologous with the ei)ipodial folds of 

 a gastro})od, the arms must be regarded as independently acquired 

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

 arms nor of the sii)hon 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 in the cephalopods. 



COMPAEATIVE STRUCTURES OF THE TETRABEANCHIATE AND 

 DIBRANCHIATE CEPHALOPODS. 



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

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

 nautiloids and cuttle-fishes. The tetrabranchiates are undeniably a 

 more primitive and generalized type than the dibranchiates, and in their 

 adult condition exhibit characteristics which are transitory in the other. 

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



* Brooks (W. K.). Homology of the Ceplialopod Sipliou aud Arms. Am. Jour. Sci. 

 (3), vol. XX, pp. 288-291 ; also, The Development of the Squid Loligo Pealii (Lesereur) 

 Anniv. Mem. Boston Soc. Nat. Hist. 



