1874.] Developmental History of the Mollusca. 237 



nearly representing the molluscan type than do the other classes, or, in 

 other words, more closely resemble the ancestral forms than they do, we 

 might look, in the course of the development of the less typical Mollusca, 

 for some indication of a representative of the internal pen of the higher 

 Cephalopoda. We might expect to find some indication of the connexion 

 between this and the calcareous shell of other forms ; in fact the original 

 shell of all Mollusca should be an internal one, or bear indications of a 

 possible development into that condition. 



In Pisidium, in Aplysia, and in Neritina the author has submitted evi- 

 dence of the existence of a specially differentiated patch of epidermic 

 cells at the aboral "pole, which develops a deep furrow, groove, or pit in 

 its centre almost amounting to a sac-like cavity opening to the exterior. 

 The first (chitinous) rudiment of the shell appears as a disk on the sur- 

 face of this gland ; but also, in some cases, the cavity or groove is filled by 

 a chitinous plug. 



Let the walls of the sac close and the activity of its lining cells con- 

 tinue, and we have the necessary conditions for the growth of such 

 a " pen " as that of the Decapodous Cephalopods. 



At present the details of the development of the " pen " in the Cepha- 

 lopoda are not fully known ; but the author has evidence that it is formed 

 in an enclosed sac-like diverticulum of the epidermis, but he has not yet 

 ascertained the earliest condition of this sac. The history of its develop- 

 ment becomes surrounded with additional interest in relation to the shell- 

 gland of the other Mollusca. 



The position of the groove of the shell-gland in Pisidium suggests a 

 possible connexion of its chitinous plug with the ligament, which it will 

 be worth inquiring into in other developmental histories of Lamellibranchs. 

 The internal shells of other Mollusca besides the cuttlefish are cer- 

 tainly not in some cases (e. g. Aplysia} primitively internal, but become 

 enclosed by overspreading folds of the mantle. But in the case of Limax 

 and its allies, it is possible, though the matter requires renewed investiga- 

 tion, that the shell is a primitively internal one representing the shell- 

 plug. 



There is yet one more possible connexion of this shell-gland and plug : 

 this is the chitinous secretion by which Terebratula and its allies fix 

 themselves to rocks &c. The position of the peduncle exactly corre- 

 sponds to that of the shell-gland; and an examination of Professor 

 Morse's recently published account of the development of Terebratulina 

 leaves little doubt that at the pole of attachment, which very early deve- 

 lops its function and fixes the embryo, an in-pushing occurs, and a kind 

 of shallow gland is formed which gives rise to the horny cement. The 

 author's own observations on the development of Terebratula vitrea do 

 not extend to so early a period as this. 



It is perhaps scarcely necessary, in conclusion, to point out the close 

 resemblance of shell-gland and plug to the by s sal gland and its secretion. 



