1902.] On the Gastric Org mis of Spirula, Nautilus, etc. 235 



spicuous folds (s.f.) which lead into a convoluted valvular arrange- 

 ment, which projects beyond the parietes of the gastric tube. Between 

 these folds the duct of the immense liver (b.d.) of Nautilus opens, just 

 as the duct opens between the folds leading into the spiral caecum of 

 Nassopsis or Pleurotomaria among the Gastropods (compare figs. 1 and 

 2). Exactly the same results are obtained, but in even a more striking 

 manner, if we compare the stomachic apparatus of Spirula (fig. 4) with 

 that of Nassopsis.* In this case the stomach proper is far better 

 developed than in Nautilus or Sepia (fig. 3, st.), and the corre- 

 spondence between the different parts in the Cephalopod and the 

 architaenoglossan Gastropod is at once clear and striking. 



From these observations it would appear that if we take Nassopsis 

 as a Gastropod, and either Spirula or Nautilus as a Cephalopod, there 

 is nothing incomparable in the stomachic apparatus of these widely 

 divergent molluscan forms ; on the contrary, they are exactly compar- 

 able in all their main features, and it would consequently appear to 

 follow, firstly, that in both cases we are dealing with an extremely 

 primitive type of organisation, and, secondly, that Nautilus among the 

 Cephalopods and Nassopsis among the Gastropods retain a type of 

 gastric apparatus which must have been possessed by the common 

 ancestors of both.f 



The different kinds of modification which this primitive gastric 

 apparatus has undergone in the more specialised molluscan groups 

 may be exemplified by what is found in the following types : — In 

 Nassopsis, Spirula, Nautilus, and the Cephalopods generally, there is 

 both a style-sac and a csecum. In the Prosobranchiate Gastropod 

 Paramelania there is a style-sac and a rudimentary caecum. In Tur- 

 ritella communis there is a style-sac, and only a portion of a csecum. 



* In comparing the stomach of Spirula with other forms, we have been able 

 to refer to the late Professor Huxley's unpublished notes and drawings relating 

 to this and other Cephalopods preserved in the Roy. Coll. of Science, and it is 

 most interesting to find that of all the Cephalopods examined the gastric apparatus 

 in Spirula shows the closest similarity to that of Nassopsis and Limnotrochus 

 among the Grastropods. Huxley always inclined to the belief that Spirula is the 

 living representative of the Belemnites, and the above comparisons would certainly 

 seem to show that it retains a very primitive alimentary canal. 



f It is worthy of mention that the forms of Tsenioglossan Prosobranchiata, which 

 retain both the style-sac and the csecum, if conchological deductions can be trusted, 

 are among the very oldest of fossil forms. The Capulidse, to which Limnotrochus 

 and Chytra are allied, extend back into the Cambrian, and the same may be said 

 of the Pyramidelidse, which appear to be the ancient representatives of the Turri- 

 tellidse of the present day. Nassopsis, on the other hand, is more clo£ely related 

 to forms which, like Ampullaria, unite the characters of the Tsenio- and Rhipido- 

 glossate types. All the Cephalopods are much later in origin, and it would appear, 

 as so often happens, that they retain an archaic character which the living repre- 

 sentatives of the older diatocardiate Prosobranchs, such as Pleurotomaria, have 

 partially lost. 



