THE DEVELOPMENT OF THE UNIONIDAE. 49 



and useless afterwards, and has almost entirely disappeared from 

 even the embryonic stages of growth (JACKSON). In Pecten* another 

 Myomyurian, on the contrary, the foot is very well developed in the 

 nepionic period, and serves as a locomotory organ ; during metamor- 

 phosis it becomes considerably reduced though, with its byssal gland, it 

 is still present in the adult, but no longer serves as a locomotory organ, 

 since the animal now swims by clapping its shell-valves together. 

 In other Lamellibranchs also, viz., those forms which spend the 

 greater part of their life attached to some foreign body, e.g., Dreis- 

 sensia polymorpha, the foot which, at first, is very large (Fig. 20, 

 p. 42), becomes very much reduced in size as the animal develops. 

 After the velum has degenerated and before attachment, Dreissensia 

 passes through a stage in which it creeps about very actively with 

 the help of its foot (No. 27). This form at a later stage also occa- 

 sionally moves about, but, in consequence of the great reduction 

 of the foot, its movements are very slow (No. 58). 



[In those Lamellibranchs which, in the adult stage, lead a fixed 

 life, attached by means of a byssus to the substratum, the portion of 

 the foot carrying the byssal gland is retained, although the loco- 

 motory function of the foot may be completely lost, e.y., Anomia. 

 In Ostrea, where the attachment is brought about by a secretion of 

 the left mantle-lobe, all trace of the foot is lost in the adult.] 



5. The Development of the Unionidae. 



The development of the Unionidae differs so essentially from that 

 of the other Lamellibranchia that, except with regard to the cleavage 

 of the egg, it must be treated separately. It has evidently under- 

 gone radical modification through change of the external conditions 

 of life, and the whole of its later development is no doubt influ- 

 enced by the assumption of a temporary parasitism by the young 

 or larvae, which become attached to the gills or to the integument 

 of fishes. We thus find, in the Unionidae, superadded to the normal 

 course of development, as observed in the marine Lamellibranchs, an 

 additional and unique larval form which cannot be compared with 

 the larva of the latter, and which possesses characters not present 

 in the adult. 



* The development of Pecten has been investigated by FULLABTON, from 

 whose treatise, which is illustrated by four plates (No. 14), we gather that 

 this form develops exactly like other marine Lamellibranchs up to the later 

 larval stages. The transformation of the larva into the adult was not observed. 



E 



