THK SENSOKY ORGANS. 295 



only when the degeneration of the shell has reached its highest limit (horny 

 shell of the Dibranchia). We might presuppose a similar process in the 

 ancestors of Anjouantd, and thus claim the shell which occurs only in the 

 female and is altogether wanting in the male, as a new formation.* 



The only Cephalopod provided with an external shell, the ontogeny of 

 which is at present known, is thus not adapted to assist in the solution of 

 the problem as to the significance of the shell-gland ; we are therefore 

 restricted to the embryological facts concerning the forms with internal 

 shell. 



Since the invagination known as the shell-gland secretes the whole shell, 

 and therefore also the part of it which corresponds to the rostrum of the 

 Belemnites, and as this is a part added to the primitive (Ammonite or 

 Nautilus) shell, there can be no doubt that the invagiuation is the equivalent 

 of at least that part of the mantle-sac which covers the internal shell. The 

 extraordinarily early appearance of the invagination indicates, however, that, 

 in the Cephalopoda as in other Molluscs, the original shell owes its origin to 

 a shell-gland. If this is the case, we might assume that in the course of 

 development the shell-gland became connected with this secondary mantle- 

 sac. The position of the primitive invagination is easily reconcilable with 

 this assumption. This question can be satisfactorily answered only when the 

 formation of the shell in recent Cephalopods with external shell such as 

 Sinnilti and above all Nautilus, is understood. Since the first of these forms 

 is closely allied with the Decapoda and since, from the examination of the 

 ovaries in the latter (OWEN, No. 33) it is known that, like other Cephalopoda, 

 it has large eggs rich in yolk, it is not too much to assume that similar 

 processes of development took place in these forms also, the conditions 

 of development of the shell especially being similar. 



C. The Sensory Organs. 



The sensory organs of the Cephalopoda (olfactory, auditory and 

 visual organs) show much similarity in their development, all appear- 

 ing first as imaginations of the ectoderm. The olfactory organs are, 

 throughout life, mere ectodermal invaginations, but the auditory 

 organs and especially the eyes become partly or altogether separated 

 from the ectoderm and reach a high degree of development. In 

 these cases also, however, traces of the connection with the ectoderm 

 may be retained either as vestigial or specialised structures (as in 

 the auditory organs) or by the retention of the original aperture of 

 invagination, this latter condition being exhibited in the eye of 

 Nautilus. 



* Cirrhoteuthix is said, unlike other Octopoda, to possess a shell, the nature 

 of which, however, is not well understood. If it is a true shell, it no doubt 

 arises from a shell-gland as in the Decapoda, and we should be justified in 

 assuming that forms like Octopus and Argonauta, in which a shell-gland 

 occurs, once possessed vestigial shells. The case inhabited by Argonauta 

 could then no longer be homologised with a true Cephalopodan shell. 



