Homologies of the Cephalopoda. 311 



The second objection is, that the nerves of the tentacles arise 

 independently from the ganglia, and each one is therefore 

 homologous to a single arm, the rest having aborted. But as 

 each sucker of the Argonaut, as shown by Beneden*, has its 

 ganglion and nerve, it is these that are homologous with the 

 several tentacles of Nautilus, which each have a single nerve 

 arising from a ganglion, not yet separated at the base — the 

 only difference being that each tentacle is separate, and the 

 development being from the multiple to the simple, the suckers 

 on the Dibranchiates are collected onto an arm whose nerve 

 though gangliated is single. The other two objections need 

 not be noticed, as they have been answered by implication ; 

 but there is an argument in favour of this homology derived 

 from knowledge acquired since the time of that paper. One 

 of the most remarkable features of the Dibranchiates is the 

 liectocotylization of one of the arms of the male, whereby it 

 is made an organ subservient to reproduction, though there 

 is no constancy with respect to the particular arm which under- 

 goes this change. Now Van der Hoevenf has shown, and 

 Keferstein J has confirmed the fact, that the male Nautilus in 

 like manner suffers liectocotylization, by which the organ 

 called the spadix is produced, an organ which, like that of the 

 Argonaut and others, has a glandular function, and is brought 

 into relation with the spermatophores. Now if each tentacle 

 were homologous to an arm it should be one of the tentacles, 

 or part of one, which is so modified. But what is the case ? 

 In the female the corresponding labial process is divided into 

 two parts, one supporting four tentacles and the other eight ; 

 and it is the corresponding four tentacles in the male which 

 make up the spadix within which they may be seen in trans- 

 verse section. Thus it is part of one of the processes whose 

 tentacles are modified in the Nautilus, just as it is one of the 

 arms whose suckers are modified in the Octopod ; ergo the 

 process is the homologue of the arm. 



Again, there is a word to say about the hood of the Nau- 

 tilus. On account of there being a tentacle contained in a cavity 

 on each side within, this has been taken to represent the 

 two foremost tentacular processes, or, as I may now call them, 

 arms of great substance, and which have grown together; and 

 so the eight arc made out. Whether this is the right way to 

 look at them, or whether the hood is not an independent organ 

 which has grown to the single-tentacled arms lying immedi- 



* "Memoire sur l'Argonaute," Acad. Brussels, vol. xi. 1838. 

 t Wis. eu Natuurk. Verb, der Koninkl. Akad. deel iii. 1856; anil Ann 

 & Mag. Nat. Hist. 1856. 



| Bronn's ' Klassen und Ordnungen,' Band iii. L865. 



