DEVELOPMENT OF THE EXTERNAL FORM PTEROPODA. 169 



margin, but the latter is marked off by a constriction from the part 

 which represents the adult shell ; here also, in Caoolinin t a transverse 

 wall is secreted, after the body of the animal has withdrawn from 

 the posterior part of the shell. This larval shell is afterwards lost. 

 In other Cuwtlimidae, the larval shell is retained even in the adult 

 (titt //iota), the posterior part of the body not being withdrawn from 

 it (Ct'fi#ei#). The coiled larval shell of the Limacinidae passes directly 

 over into the adult shell, new coils merely being added to those 

 already present (Liniadna, Spirialis).. In the Cymbuiiidae, the 

 larval shell can hardly be distinguished from that of the young 

 animal undergoing metamorphosis. This calcareous shell is thrown 

 off, the cartilaginous shell of the adult surrounded by the mantle 

 then appearing ; this shell arises by the thickening of the connective 

 tissue and can therefore not be in any way compared to a true 

 Molluscan shell (PELSENEER). 



The transformation of the shell just described is one of the most 

 conspicuous features among the external alterations undergone by 

 the larva. In the Cavoliniidae, the shell lengthens, and, in the Ct/m- 

 l>nln<!a<' and Limacinidae, becomes rolled up (Fig. 75 A and B). 

 Even in the straight shells of the Cacoliniidae we find a slight flexure 

 of the posterior end which gives the shell the shape of a hunting 

 horn. It is a curious fact that the concavity of this slightly bent shell 

 does not correspond to the ventral side, but lies dorsally. This must 

 be connected with a twisting undergone by the posterior part of the 

 body in these forms (BoAs, Nos. 9 and 10). The coiled shell in any 

 case represents the more primitive condition and persists throughout 

 life in the Limacinid/ie, which are also provided with an operculum. 



The development of the foot exercises a great influence on the 

 changes that take place in the external form of the body. The foot 

 originally was a large projection lying behind the mouth. While 

 the middle part of the foot does not increase greatly in size, and at 

 first is conical or linguiform, two projections arise at its sides and 

 grow out rapidly (Fig. 74, pit, and 75 A,Jl) in the form of two large 

 lobes, the so-called fins (Fig. 75 /*, ft). The great size which may 

 lf attained by the fins in the further course of metamorphosis is 

 already sufficiently known. The median lobe of the foot also in- 

 creases in size. In the Cymbtdiidar, a filiform appendage develops 

 on it posteriorly. Ontogeny proves indisputably that the fins owe 

 their origin to the foot, as was observed long ago by JOH. ML)LLER 

 and KKOHN. 



The I'' //,/,/ larva of the Pteropoda shows great agreement with 



