[12 FESTSKRIFT FÖR LILLJEBORG ig 



hind-end we already have indications of a rostrum to come; a shell with such a struc- 

 ture as that ol Belemnoteuthis with its low chambers could hardly be rolled up and 

 Spirula-like. Moreover, Belemnoteuthis had hooks and thus was more specialised, 

 for although it is easy to understand how through one-sided development of the arma- 

 ture of the suckers such organs can be transformed into hooks 1 , the reverse process 

 is altogether improbable. According to my opinion, a hooked form can be derived 

 from one with acetabula but not the reverse and if we take into consideration the 

 shape of the Spirilla suckers, we shall see that these with their large outer margin 

 or "collar", their uniform dentition all round and their comparatively small opening 

 to the interior lumen have a form which makes them seem more remote from hooks 

 than the suckers of most other Decapods. As Spirula cannot have originated from 

 Belemnoteuthis it cannot be derived from any other of the known dibranchiates, fossil 

 or recent, that is to say, from dibranchiates according to the present classification. 



We find from Appellöf's paper" on the shells of Sepia, Spirula and Nautilus 

 that the system of growth is about the same in these so different shells and that the 

 differences are mainly due to the position and the function of the shells. It is also 

 clear that the shell of Spirilla's ancestors has once upon a time been an outer shell 

 1 at the same time that it must have been subject to the same changes in struc- 

 ture, when it became wrapped up in the pallium. 



Several writers have shown, that there exist not a few likenesses between 

 Spirula and the Ammonites, for instance, with regard to the first chamber, the sipho 

 etc., but others have declared that the Nautiloidea and the Ammonoidea cannot be 

 separated, because the structure of the shell of both groups is so similar. But as all 

 shells of Cephalopods are homologous (except that of Argonaula?), it is evident that 

 external shells, even if they belong to different groups, must be very much alike, 

 and for this reason the shells of the Nautiloidea may be like those of the Ammonoidea, 

 without the animals themselves being necessarily closely related merely because 

 they have a similar function. On the other hand, if a shell that has been an exter- 

 nal one, enclosing the animal completely, and perhaps also protecting it with an 

 open le ("Aptychus"), as in certain Ammonoidea, becomes itself more and more en- 

 closed within the folds of the mantle it must be subjected to several changes. It be- 

 comes thinner as it is protected by the mantle and not exposed any longer to the 

 same dangers as before. The exterior ornaments, lobes, spines etc. disappear as 



Similar development can be observed daring the ontogeny of several Onychii for instance Go- 

 naliis f,il>>iiii a. o. 



2 K.. Svenska Vet. Akad. Hindi. Bd. 25. Stockholm 1893. 



