AND ORDER IN TETRABRANCHIATE CEPHALOPODS. 207 



In other series, such as the Arietes of the Lower Lias, we have in the same formation, 

 and side by side, embryonic, adult, and old age forms, and among the Nautiloids the old 

 age forms actually precede in the Silurian the great display of the fully ornamented adult 

 forms of the Carboniferous. 



I can only claim, therefore, that there is a correspondence between the manifestations 

 of decline in the individual and in the degradational features of the group, whether these 

 appear early or late in time. 



It may be said that the smoothness of the Lrevigati in the Nautilian series is entirely due 

 to a retention of embryonic characters throughout life ; and their early advent in the Silu- 

 rian certainly affords good grounds for such a conclusion. On the other hand, however, 

 their development, omitting as it does the characteristics of the straight Endoceras and 

 Orthoceras, seems to identify them with such undoubtedly senile species as Amm. obtusus, or 

 Amm. stcllaris. The resemblance of the adult features of such species to the old age charac- 

 teristics of lower species are too clear to be accounted for by an arrest of development ; 

 unless, indeed, we can assert that the resemblance of the old and 3'oung in the same shell 

 is due to an arrest of development, which would be a manifest absurdity. 



I have been dealing so far with but one organ, the shell ; and notwithstanding the 

 utmost care and frequent reviews of the specimens themselves, I have found that the facts 

 always revealed the same laws, and ever more decidedly. But some time since I satisfied 

 myself that if the softer parts had been preserved, one of my conclusions would have re- 

 ceived important qualifications. In order to obtain an adequate illustration of this, I will 

 conclude with one or two examples from existing animals. 



Mr. Morse, besides elucidating the general homologies of the Saceata, shows with the 

 greatest clearness that the Cephalopod is very similar in form to the Polyzoon, although 

 completely reversing its structure. 1 Thus, while both are sacs closed at one end by a disc 

 perforated by the mouth and fringed with a circular crown of tentacles, and their alimen- 

 tary canals bent in a similar way and opening in the vicinity of the mouths, one is a per- 

 fectly cephalized, and the other an anti-cephalic type. The nerve-mass of the Cephalopod 

 is situated at the anterior, and in the Polyzoon at the posterior pole ; and while in the for- 

 mer the anus and intestine are on the ventral side, in the latter they are on the dorsal side. 

 By entirely revolving the positions of the principal organs a morphological polarity is es- 

 tablished in the Cephalopod ; which being the structural reverse of the Polyzoon is yet so 

 alike in shape, that either a longitudinal or transverse section of the body has the same 

 outline. The element of time being eliminated, these polarities are related morphologi- 

 cally to the intermediate members or classes, as the aberrant Ammonoids and Nautiloids 

 were to the normal forms ; they are organically removed from each other to the greatest 

 possible degree, being at the extremes of the Saccate type. 



It will be remembered, also, that the polarities were brought about by a revolution of 

 the straight cone into an involute cone, and subsequently from the involute to a straight 

 cone again. The intermediate steps of the revolution. of the organs between the Polyzoa 

 and Cephalopoda exhibit the action of the same law. 



Mr. Morse's figures in the single plate attached to his paper give us a distinct idea of 

 the successive chancres in this remarkable series of transformations. The anus remains 

 more or less posterior, and the oral region with the other organs is carried by a cephalic 



E. S. Morse, Proceedings Essex Inst. Vol. iv., p. 6, 1S65. 



MEMOIH3 BOST. SOO. NAT. HIST. Vol I. Pt. 2. 53 



