230 EMBRYOLOGY 



larva, unite to form the epidermis of the worm. Since 

 the discs, owing to their mode of origin, are bilaminar, the 

 body-covering of the worm, formed by the inner layer, is 

 enclosed by an envelope (amnion), the outer layer of the 

 discs. The larva itself goes to pieces. In the type of Besor, 

 the discs, which likewise arise from the ectoderm, are from 

 the beginning unilaminar only; the amnion, therefore, is 

 absent, whereas in other respects the developmental pro- 

 cesses are quite similar. Finally, the discs are no longer 

 formed at all. However, as a suggestion of the former 

 mode of development, the outer ectodermal layer separates 

 from the embryo and is cast off (Cephalothrix). Moreover, 

 the embryo bears rigid cilia, as in Pilidium. This is also 

 the case in those embryos which are metamorphosed directly 

 into the worm without having their ectodermal covering 

 undergo any important changes. 



Accordingly the pilidium appears as the older type of 

 development, from which the others are derived by becom- 

 ing at the same time simplified. But even the development 

 of the pilidium cannot be the original form. The origin of 

 the worm within the larva is a secondary process, which has 

 probably arisen through adaptation to the conditions of life. 

 Originally the larva was certainly metamorphosed directly 

 into the worm, as is still the case in the Turhellaria and 

 Annelida^ for example. If the statements of Fewkes (No. 5) 

 should be verified, those forms in which the pilidium is said 

 to be resorbed by the body of the worm could best explain 

 the original mode of development (comp. p. 220). 



The shape of the Nemertean larva, irrespective of the Tornana-like 

 form (Fewkes), points to relationships of two kinds^ One of these con- 

 cerns the Turhellaria. The resemblance is clear without further 

 comment, if one considers the Stylochus-larva of Goette (Fig. 80, p. 

 168). This larva exhibits the two typical lateral lobes of ihe pilidium. 

 We have shown in treating of the Turhellaria how it can be referred to 

 MuLiiEK's larva. In the comparison of larval forms caution is of 

 course necessary, and especially in the case uni,der consideration, where 

 the first stages of development in the two groups differ very much from 

 each other. Thus one might be inclined to maintain the similarity in 

 the outward shape to be accidental, if the adult animals did not also 

 possess many similar characters in their organization. 



