THE TRANSFORMATION OF THE LARVA INTO THE ADULT. 97 



At a somewhat later stage, at which the velum is still retained, the 

 foot is found protruded from the shell (Fig. 39 A). This stage, as 

 well as the younger one depicted in Fig. 38 B, recalls that stage 

 in the Lamellibranch larva in which the larval and the adult organs 

 of locomotion are present and functional at the same time (Fig, 20, 

 p. 42). At the posterior end of the larva, an early specialisation 

 of the mantle-folds produced a well-marked channel, lined with 

 powerfully ciliated cells (Fig. 38 and 39, nine]. This ciliation is 

 connected with the circulation of the water, which is further pro- 

 moted by the ciliation of the mantle-cavity. 



The foot, as already mentioned, lies in front of the oral aperture. 

 It is here that the prominences arise which give origin to the 

 tentacles (Fig. 39 #, t). According to LACAZE-DUTHIERS, there are 

 at first three of these, two lateral and one smaller median prominence 

 (Fig. 39 B}. These structures, by lengthening, give rise to the 

 tentacular filaments which are so numerous in the adult. The 

 description given does not explain the relation of the filaments to the 

 prominences and to the oral aperture, but the condition of the 

 tentacles in the adult enables us to form some conclusions on this 

 subject. In the adult, the mouth lies surrounded by leaf-like labial 

 appendages at the apex of an egg-shaped projection which, together 

 with the tentacular filaments that are innervated from the cerebral' 

 ganglion, must be regarded as the cephalic region. The tentacular 

 filaments arise from two lobes lying at the base of the cephalic 

 projection, so that here also, there are three prominences which 

 might be traced back to those found in the larva. We should then> 

 as in the Gastropoda, consider the middle prominence as the rudiment 

 of the oral cone, and the lateral prominences as the two original 

 tentacles, from which later the tentacular filaments arise. 



A similar view of the tentacle-filaments of the adult is taken by THIELB 

 (Literature to Chapter xxxiii.), who compares the two lobes or tentacular 

 shields with the large tactile lobes of Haliotis which are beset with tufts. 

 These latter, if lengthened, would result in structures resembling the ten- 

 tacular filaments. Quite recently PLATE (No. 3) also has accepted this view, 

 ascribing to the three prominences on the head of the young animal the 

 above significance. 



The radular sac arises during the later stages of larval life as an 

 outgrowth of the stomodaeum. The anus also appears in the larva 

 as a slight depression of the ectoderm behind the base of the foot. 

 The enteron, according to KOWALEVSKY, becomes connected with it 

 direct, without the formation of an ectodermal rectum. 



H 



