MR. T. H. HUXLEY ON THE MORPHOLOGY OF THE CEPHALOUS MOLLUSCA. 45 
But in any natural group of animals the grand laws of development and growth 
are so uniform (tlie uniformity in fact constituting the true bond of union of its 
members), that this want may be supplied by the very full information we possess 
with regard to other Mollusca. If from these data certain general propositions can 
be established, it will, I think, be perfectly fair to make these propositions the basis 
whence deductively to explain and account for facts of organization whose absolute 
genesis is not known. 
The development of the Cephalopoda, Pulmonata, Nudibranchiata, and Tecti- 
branchiata, has been very carefully made out by Kolliker, Van Beneden and 
W iNDiscHMANN, ScHMiDT, Gegenbaur, Sars, Nordmann, Vogt, Reid, and others. 
From their observations the following generalizations may be very safely made. 
1. The development of a Mollusk 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. fig. 1. 
3. 4 he subsequent bent, spiral, or otherwise unsymmetrical arrang'ement of the 
parts of the body in Mollusca, depends upon the development of one part at the 
expense of, or disproportionately to, another; and this asymmetrical over-develop- 
ment never affects the head or the foot of a Mollusk, but only a portion, or the 
whole of the limmal surface. Plate V. figs. 2-8. 
4. It is to this portion, and its often free projecting edges, that we can alone pro- 
perly 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, a post-abdomen. 
5. All embryological evidence goes to show that the Cephalopoda and Pulmonata 
develope an abdomen. The intestine becoming drawn into the abdominal sac 
becomes in consequence bent towards the neural side. Plate V. figs. 2-5. 
6. On the other hand, all the evidence hitherto obtained with regard to the de- 
riiis very remarkable law has not, it appears to me, received its due importance at the hands of those 
distinguished anatomists, Kolliker (for the Ceidialopoda), Van Beneden and Windischmann and Gegen- 
baur (for the Pulmonata), \ ogt (for the Nudibranchiata), and Leydig (for the Pectinibranchiata), from whose 
observations I deduce it. \ ogt, however, observes that the order of appearance of organs in the Mollusca is 
the imer^e of that in the "V ertebrata; and with regard to the point from whence development commences, he 
sajs, Ce point est facile de trouver, il est situe en arriere des roues a peu pres sur la ligne de junction 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 \ ertebrata and the Invertebrata. The hsemal side is that upon which the vas- 
cular 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 Vertebrata, the ventral of Invertebrata. 
t Instead of the radial type of development we meet quite unmistakeably 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. 
MDCCCLIII, u 
