Development of the Pectinibranchiata. 437 



der, which is twisted to form loops ; cilia are seen on its inner 

 margin. It afterwards becomes a little flattened, at the same 

 tnne dilating considerably. In its walls we discover longitudinal 

 and transverse fibres, and the cilia of the middle of each loop 

 are of extraordinary length. After the production of the bran- 

 chiae it becomes extremely difficult to investigate the formation 

 of the other organs ; on the one hand because the animal rarely 

 elongates itself sufficiently out of its shell to allow its parts to 

 be perceived, and, on the other, because the mantle has become 

 a good deal thickened, and the shell has been the seat of a con- 

 siderable deposit of calcareous matter. This shell has acquired 

 the form of that of a Nautilus. The rotatory organs diminish 

 considerably in size. The foot, which is lobate above, acquires 

 more and more the form of that of the adult animal. The oper- 

 culum, which serves to close the aperture of the shell, is com- 

 pletely developed. The heart, in this stage, is divided into two 

 chambers. The lenses of the eyes are clearly distinguished : we 

 have pretty frequently found a single eye which presented two 

 projections of pigment, each provided with a lens. 



The branchial cavity, of which the inner surface is clothed 

 with cilia, has become, at this period of development, sufficiently 

 deep to contain the heart entirely. The margin of the mantle 

 which is furthest removed from the body of the animal is fur- 

 nished with cilia, and at the bottom of the branchial cavity we 

 for the first time discover a contractile vesicle (kidney), similar 

 to that which exists in Buccinum undatum. After the lapse of 

 eight weeks, the young animals have not yet quitted the cap- 

 sules, and when one is taken out in this state, it sets itself to 

 creep like the adult animal, with the foot, the tentacles, and the 

 siphon extended. It is then distinguished from the adult in 

 that the rotatory organs have not entirely disappeared, that the 

 shell is not hard, and also by the spire, which has only one, or 

 at most, two turns. During the ninth or tenth week the young 

 animals quit the capsules; the rotatory organs have then dis° 

 appeared, and behind the tentacles we observe a raised line, 

 which indicates the place previously occupied by them. 



The shell has become more elongated, and approaches nearer 

 to that of the adult ; it is hard, brittle, and nearly opake, but 

 the last turns of the spire are not yet developed. We have not 

 referred to the way in which the development of the organs takes 

 place, because it does not differ from that which occurs in Buc- 

 cinum undatum. But, before concluding our memoir on the de- 

 velopment of the Pectinibranchiata, we find that it is necessary 

 to make a few observations on the memoir of Dr. Carpenter on 

 Purpura lapillus, as the results which he has obtained are very 

 different from ours. 



