416 YroL^.JjOven on the Development of the Chitons. 



The anterior shells are earlier iu acquiring their form than the 

 posterior ; thus the foremost will have assumed its crescent 

 shape before the posterior is scarcely formed. 



In the second place, we find that the shells first make their 

 appearance in the form of narrow plates with irregular waved 

 edges, and increase both in breadth and thickness by the depo- 

 sition of new and somewhat larger plates beneath those first 

 formed. But fig. 6 shows that each shell soon acquires two 

 deep notches [incisures laterales, j\Iidd.), one on each side of 

 the anterior margin. When the new laminae are deposited by 

 the mantle, these notches are gradually closed when viewed 

 from above, and only a mark on its inner part is left ; but it is 

 more than probable that by this mode of growth the lower 

 surface of the shell, which is applied against the accustomed 

 surface of the mantle, ought to present a pitted furrow directed 

 forwards and outwards {sutura lateralis porosa, Midd.). It also 

 appears that the articulamentum of Middendorff is first formed. 

 I saw no distinct indications of a tegmentum. It would seem 

 moreover that, at least in Chiton marginatus, the shells are 

 not united by four articuli, and still less is there any support 

 for the opinion that the posterior valve is the true shell, ana- 

 logous to that of Patella, and that the anterior ones are laid 

 over this. 



As regards the edges of the mantle, I have only to observe 

 that its pointed tubercles appeared quite irregularly, as they 

 were seldom present over the whole surface, but only in 

 patches. 



Nothing could be ascertained with regard to the internal 

 anatomy, from the want of transparency of the external parts. 



If we compare this development with that of other Mollusca, 

 it is evident that the circle of cirri, by means of which the 

 animal moves in its first or swimming stage, corresponds with 

 the cirri of the velum in the young of other Gasteropoda and of 

 the Acephala. But in Chiton the velum is not developed into a 

 broad, extensible sail. Instead of this, another part has acquired 

 a considerable thickness, namely, the anterior conical portion 

 having the tuft of filaments. This is exactly what I call the 

 "pyriform body" which bears the "flagellum" in the marine 

 Acephala. 



The velum disappears in many Mollusks, so as to appear only 

 as buccal tentacula or labial palpi. Perhaps a vestige of it is 

 to be found in the fold of skin which surrounds the head in 

 Chiton. 



