18 AMPHINEURA. 



This section of the body is called by PBUVOT the mantle-section in the 

 older larvae (B), and this name seems to be justified by its relation 

 to the adult body which is in process of formation. Between the 

 process of invagination just described and the formation of the adult 

 body, various other formative processes take place, but these apparently 

 occur within the larval body, and have so far not yet been investi- 

 gated. Essential differences are to be found between the development 

 of Dondersia and that of Chiton; although, in the latter also, it is 

 chiefly the part of the larval body lying behind the ciliated ring that, 

 by its elongation, becomes changed into the body of the adult. The 

 pre-oral part of the larva (i.e., the part lying in front of the ciliated 

 ring) is here retained, as in Chiton, and yields the corresponding part 

 of the adult body. It appears that, in Dondersia banyulcnsis, there 

 are two lobe-like projections at the anterior end of the body, and that 

 these are already distinct in the young animal (Fig. 10 (7). The 

 mouth is said to be wanting until metamorphosis commences; the 

 entoderm is present in the form of a solid mass, at the sides of which 

 two mesoderm-bands lie. This latter condition would agree with 

 that described for Chiton (p. 7), and is interesting inasmuch as 

 the mesoderm-bands in other Molluscs are not usually so distinct. 

 Unfortunately, no further details are known either of the internal or 

 of the external development of Dondersia. It should, however, i>e 

 stated that seven calcareous plates are found on the back of the young 

 animal which has only just undergone metamorphosis (Fig. 10 (7), 

 these being formed of rectangular spicules arranged one behind another. 

 The resemblance of this stage to a young recently metamorphosed 

 Chiton is striking, the latter also possessing seven such plates. In 

 what way this condition can be reconciled with that of the adult 

 Dondersia we do not know ; this can only be made clear by the 

 investigation of transition -stages. The young animal has lateral 

 spicules which are plate-like and imbricated, and which, in the adult, 

 seem to be less developed, as far at least as can be judged from the 

 statements as yet made on the subject. 



The form of the larva of Dondersia, like that of the Chiton, may be 

 traced back to that of the Trochophore. We have already pointed 

 out various specially striking points of resemblance between the 

 development of Dondersia and that of Chiton. The elongation of 

 the posterior section of the body recalls here almost more than in 

 Chiton the growing of the post-oral section of the Annelidan Trocho- 

 phore into the trunk of the worm (Vol. i., Fig. 120, p. 269). This is 

 specially striking if we examine the Mitraria larva (Vol. i., Fig. 124, 



