FORMATION OF THE SHELL. 409 



The cephalopodous Mollusca form no exception ; their 

 bone, composed of two or three calcareous plates, being 

 found fully developed in the egg of the Cuttle-fish some 

 time before the young animal is hatched. 



These observations are directly at variance with the theory 

 maintained by the late Sir Everard Home,* viz., that the 

 shell of the Yermes Testacea is formed after the animal has 

 quitted the egg ; and as regards the Cuttle-fish, they are 

 opposed to the remark, made by the Baron Cuvier, that the 

 young Cuttle-fish, when first hatched, has only a cartilagin- 

 ous plate like the Loligo. 



The shell when first observed on the embryo (even of the 

 animals of spiral shells) forms a short, blunt, more or less 

 curved, subcylindrical cone, covering the hinder part of its 

 body : as the organization of the embryo becomes deve- 

 loped, and the hinder part of the body extended, the shell 

 increases in size, till the body and shell together occupy 

 nearly the whole of the egg. While inclosed in the egg, the 

 embryo shells are generally of a pale horn colour, and desti- 

 tute of markings : when, therefore, they remain attached to 

 the apex of the spire of adult shells, they may be easily 

 distinguished by their appearance from the part formed after 

 their exclusion ; and as, in such cases, they offer some cha- 

 racters of importance, it has been proposed to designate them 

 by the name of the nucleus of the shell. 



The effect of the atmosphere on the shell is almost instan- 

 taneous : in some young Helices and in a species of Voluta 

 in my collection, the very first line of calcareous matter 

 deposited after their exclusion from the egg is marked nearly 

 as the adult shells of the species. 



The nucleus may be generally distinguished from the per- 

 fect shell by the rapid enlargement of its whorls, by its 

 extreme tenuity, by its want of colour, and by the great 

 obtuseness of that part which is earliest formed and consti- 

 tutes the extremity of its first volution. It is necessary to 

 pay attention to these particulars, inasmuch as the nuclei of 

 some large species have been mistaken for full-grown shells, 

 and vice versa. Thus, the Murex decollatus of Pennant is 

 the just-hatched shell of the Fusus despectus ; Risso's genus 

 Orbitina is established on the nuclei of two land shells ; and 

 the genus Vitrina was regarded by Montagu as the nucleus 

 of the common snail. In some instances the first half -whorl 

 of the nucleus (the part first formed on the embryo), instead 

 of being regularly curved, is bent across the tip of the other 



* Philosophical Trans. 1817, p. 229. 



