172 ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA 



into the abdominal sac becomes in consequence bent towards the 

 neural side. Plate V. [Plate 20] figs. 2-5. 



6. On the other hand, all the evidence hitherto obtained with 

 regard to the development of the Nudibranchiata, Tectibranchiata, 

 and Pectinibranchiata, tends to the conclusion, that in them the 

 visceral mass is thrust out behind the anus ; is in fact a post-abdomen} 

 Plate V. [Plate 20] figs. 6-8. 



A little consideration will show that the intestine drawn into this 

 must become bent towards the hcenial side, as in fact it is in the 

 embrj'os of all three groups.^ 



Upon embryological grounds, then, we should establish two great 

 primary modifications of the molluscous archetype ; the one charac- 

 terized by the development of an abdomen, and a consequent neural 

 flexure of the intestine ; the other marked by the development of a 

 post-abdomen, and the consequent Jicemal flexure of the intestine. 



But these modifications of anatomical structure exactly correspond 

 with those which I have already demonstrated, upon anatomical 

 grounds, to occur in the Pteropoda and Heteropoda ; and I trust I 

 am not overstepping the bounds of legitimate analogy in assuming 

 that the anatomical fact of a neural flexure indicates the embryo- 

 logical development of an abdomen ; that of a hsmal flexure, the 

 development of a post-abdomen ; and that therefore the Pteropoda 

 fall into the same category with the Cephalopoda and Pulmonata ; the 

 Heteropoda into that of the Pectinibranchiata, Tectibranchiata, and 

 Nudibranchiata. 



It is i"emarkable that, as regards the flexure of the intestine, similar 

 contrasted modifications of the archetype take place in those animals 

 which are the nearest allies of the Mollusca ; I mean the Ascidians 

 and Polyzoa, the Molluscoides of Milne-Edwards. In each of these 

 groups the intestine is always bent upon itself ; but while in the 

 Ascidian the bend is always hcemal, in the Polyzoon it is neural. The 

 latter fact is evident to any one who will examine a Polyzoon ; the 

 former may seem at first sight to be contradicted by the circumstance, 

 that the ganglion in the Ascidians is placed between the cloaca! and 



^ See particularly Leydig, Ueher Pahidina vivipara, Siebold and KoUiker's Zeitschrift, 

 1850, where the thrusting forwards of the anus by the development of the mantle is par- 

 ticularly shown, p. 142. 



" A considerable change in the position of the anus takes place when the fold of the 

 mantle becomes formed and moves forward, because thereby the intestine and anus are also 

 thrust forward, and to the right side.'' 



- The development of the Pectinibranchiata cannot be said to have been carefully worked 

 out yet, with the exception of that ol Pabtdina, but what has been done tends to the con- 

 clusions above stated. 



