DEVELOPMENT OF THE PECTINIBRANCHIATA. 341 



The shell, which is membranous during the first periods of 

 the development of the embryo, excessively delicate and ovate 

 or reniform, subsequently takes on the form of that of the Nau- 

 tilus (fig. 1 1 a), but subsequently becomes, by degrees, more 

 ovoid. The calcareous particles then begin to be deposited in 

 considerable numbers, forming very obvious longitudinal and 

 transverse striae^ and the shell is much less transparent than be- 

 fore. However, it is still possible to distinguish the internal 

 organs : the heart and the bladder have divided into two cham- 

 bers, the upper of which is the smaller. When the auricle con- 

 tracts, the ventricle dilates, and vice versa. A very strong muscle 

 is also observable, arising from the internal surface of the shell 

 and passing to the foot (fig. 1 1 s) ; its office is to retract the ani- 

 mal into its shell. Finally, the liver appears upon the under 

 surface of the stomach ; it is ovoid, and is formed of a mass of 

 granules containing yellow pigment (fig. llr). On the inner 

 surface of the mantle a series of folds may be observed (feuillets 

 muqueuxy Cuvier), in which lie a mass of mucous crypts. 



Subsequently the little animal continues to grow ; more and 

 more calcareous particles are deposited in the shell : the mantle 

 thickens, and it becomes almost impossible to distinguish the 

 internal organs. The two rounded lobes have disappeared, but 

 behind the tentacles we see a linear eminence indicating the 

 place they occupied. The shell has assumed a horn-yellow co- 

 lour; it becomes hard, brittle, and only semitransparent. It 

 was commonly in this state that the young left its capsule, after 

 a residence of at least eight weeks there, creeping around the 

 vessel in which it had been kept, with tentacles, foot, and siphon 

 protruded. The little whelks are now distinguishable from the 

 adults, only by their shell having not more than one or two 

 spiral turns. We may add, that we have found no traces of 

 generative organs in these young. The ova, grouped together 

 in considerable numbers, occupy the posterior part of the shell. 



2. Pur pur a lapillus. 



The ovigerous capsules are not unlike a little flask whose 

 rounded end is directed upwards and its delicate neck, by the 

 extremity of which it is attached to stones or other bodies, down- 

 wards (PL XI. fig. 1). Each capsule is hermetically closed, and 



