ON THE MORPHOLOGY OK THE CEI'HALOUS MOLLUSCA 171 



I think, be perfectly fair to make these propositions the basis whence 

 deductivel)' to explain and account for facts of organization whose 

 absolute genesis is not known. 



The development of the Cephalopoda, Pulmonata, Nudibranchiata, 

 and Tectibranchiata, has been very carefully made out by KoUiker, 

 Van Beneden and Windischmann, Schmidt, Gegenbaur, Sars, Nord- 

 mann, Vogt, Reid, and others. From their observations the following 

 generalizations may be safely made. 



1. The development of a MoUusk commences on the haemal'^ side, 

 and spreads round to the neural side, thus reversing the process in 

 Articulata and Vertebrata. 



2. In all Mollusks the axis of the body is at first straight, and its 

 parts are arranged symmetrically with regard to a longitudinal 

 vertical plane, just as in a vertebrate or an articulate embryo.- Plate 

 V. [Plate 20] fig. I. 



3. The subsequent bent, spiral, or otherwise unsymmetrical arrange 

 ment of the parts of the body in Mollusca, depends upon the develop- 

 ment of one part at the expense of, or disproportionately to, another ; 

 and this asymmetrical over-development never affects the head or the 

 foot of a Mollusk, but only a portion, or the whole of the haemal 

 surface. Plate V. [Plate 20] figs. 2-8. 



4. It is to this portion, and its often free projecting edges, that we 

 can alone properly apply the term " mantle!' When this outgrowth 

 takes place before the anus, I propose to call it an abdomen ; when it 

 takes place behind the anus, 2. post-abdomen. 



5. All embryological evidence goes to show that the Cephalopoda 

 and Pulmonata develope an abdomen. The intestine becoming drawn 



1 This very remarkable law has not, it appears to me, received its clue importance at the 

 hands of those distinguished anatomists, Kdlliker (for the Cephalopoda), Van Beneden, and 

 Windischmann and Gegenbaur (for the Pulmonata), Vogt (for the Nudibranchiata), and 

 Leydig (for the Pectinibranchiata), from whose observations I deduce it. Vogt, however, 

 observes that the order of appearance of organs in the Mollusca is the inverse of that in the 

 Vertebrata ; and with regard to the point from whence development commences, he says, 

 " Ce point est facile de trouver, il est situe en arriere des roues a peu pres sur la ligne de 

 jonction entre la partie cephalique et la partie ventrale, et meme un peu en arriere de cet 

 derniere sur la partie abdominale meme," p. 39. 



I use the terms hamal and neural here to avoid the ambiguity of dorsal and ventral, which 

 have opposite meanings when applied to the Vertebrata and the Invertebrata. The hcemal 

 side is that upon which the vascular centre is developed, it is the dorsal side of Articulata, 

 the ventral of Vertebrata. The neural side is that upon which the nervous centres are 

 developed ; it is the dorsal side of \'ertebrata, the ventral of In\ertebrata. 



" " Instead of the radial type of development we meet quite unmistakably with a lateral 

 symmetrical type ; instead of the extended form of the body we find a short compressed body 

 without repetition of segments or lateral appendages."— R. Leuckart, Morphol. p. 125. 



