174 'JN THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



Of all mollusks A tiania possesses the best developed /^/-proper, 

 and has its parts best specialized and separated. In the special de- 

 scription of Atlanta names have been given to these parts, whose 

 appropriateness is, I hope, obvious. 



From this highly developed condition of the foot, to its diminished 

 state in Glaucus and its total absence in Phyllirrhoe, we have every 

 gradation. The various portions are still to be distinguished in 

 Pteroccras and the StroinbidcE, but lose their distinctness for the most 

 part in other Gasteropoda. However, the propodium is still marked 

 off by a transverse fissure in Oliva and Ancillaria, and attains a great 

 development in size ; a peculiarity which is still more remarkable in 



Leuckart, for iastance (op. cit. pp. 155-59), considers that the anterior cephaUc lobes 

 of the embryo Cephalopod answer to the cephalic velum of Gasteropoda ; the posterior 

 cephalic lobes to the als of Pteropoda, while the funnel corresponds with the middle lobe of 

 the foot. The arms he considers to be peculiar structures, mere appendages to the cephalic 

 lobes. 



If the halves of the funnel, however, answer to the middle lobes of the foot, how is it that 

 they unite upon the dorsal surface of the neck ? If the anterior cephalic lobes answer to the 

 vela of Gasteropoda, how is it that the latter disappear, and do not contribute to the forma- 

 tion of the head in Gasteropoda ? Finally, it must be remembered that the arms of the 

 Cephalopoda arise quite independently of the cephalic lobes, the first developed arms being 

 those most distant from the head. 



Leuckart considers that the oral lol^es of the pulmonate embryo are the homologues of the 

 ciliated vela of Gasteropoda. But their position and number are against this view. It seems 

 to me that these oral lobes correspond with the cephalic lobes of the embryo Cephalopod, and 

 it has been well shown by Gegenbaur {op. cit.] that the whole so-called "yelk-sac " of the 

 Pulmonata is the true homologue of the vela in Pectinibranchiata ; the "ciliated bands" of 

 Van Beneden and Windischmann turn out to be Wolffian bodies, and to be internal, not 

 external organs. 



The common view, that the ala; of the Pteropoda are the persistent vela of the embryo, is, 

 so far as I am aware, unsupported by any evidence. Embryology teaches us hitherto that 

 the anterior part of the epipodium is never permanently developed, and the position of the 

 alK would lead to the belief that they correspond to its lateral portions.^ 



So far as I can judge from the Latin table affixed to his Swedish essay on the development 

 of the Acephala, Loven considers the arms of the Cephalopoda, the three pairs of cephalic 

 tentacles of Clio, and the cephalic lobes of Tethys, to be homologous with the velum of the 

 Gasteropod embryo, while the funnel of Cephalopoda and the ala; of Pteropoda are homo- 

 logous with the foot of Gasteropoda. 



The considerations above cited appear to me to furnish a sufficient refutation of these 

 views, which seem to be the offspring of an idea first propounded by their learned author in 

 his " Contributions to the Embryology of the MoUusks " (Oken's Isis, 1842), that the hood 

 of Tethys and the cephalic processes of Tergipes are modifications of the cephalic vela of 

 the embryo. This ingenious hypothesis has not, however, been confirmed by observatiisi 

 so far as regards Tethys, and has been directly rfzVproved with respect to Tergipes (see 

 Schulze, Ueber die Entwickelung d. Tergipes lacimilatiis, Wiegmann's Arch. 1849). 



' The researches of Vogt, already referred to, have fully confirmed this conclusion. The 

 embryo Pteropod has vela, which eventually disappear, while the epipodia are developed 

 quite distinctly from the upper part of the sides of the " foot.'' 



