DISTOMA LANCEOLATUM. 189 



limited extent of the development of the vitelhgene glands, there 

 is little which need be added to the general and specific characters 

 previously described. 



The development of Distoma lanceolatum is, however, a matter 

 of greater importance, especially since our knowledge of its life- 

 changes suggests an interesting comparison between the earher 

 embryonic stages displayed by the larvse of difierent trematode 

 species. It is to Leuckart that we are indebted for most of the 

 facts bearing upon this subject. In the first place, it is especially 

 worthy of notice that the contents of the ova attain a tolerably 

 high degree of embryonic formation before the eggs quit the uterine 

 or oviducal canal of the parent ; consequently, it is not improbable 

 that the subsequent larval metamorphoses are accomplished with 

 more rapidity than obtains in the case of Fasciola. It would seem, 

 however, that the embryos do not make their escape from the egg- 

 coverings immediately after the eggs have been deposited by the 

 parent, but, according to the experiences of Leuckart, an interval 

 of a few weeks must elapse. In the early condition, the rudimen- 

 tary embryo occupies the centre of the egg, and always has its 

 cone-shaped, cephalic end directed towards the upper pole of the 

 shell, or, in other words, to that end which is furnished with the 

 Ud-hke operculum. In the mature state, whilst stfil within the uterus, 

 the cone-shaped head is supplied with a ciliated crown. Speaking 

 of its intimate structure, Leuckart says, " It is finely granular, and 

 armed at the tip with a dagger-hke spine, which, with the simul- 

 taneous displacement of the adjacent granular mass, can be pushed 

 forward and drawn back again." Besides this so-called cephalic 

 granular mass, there are within the embryonic body two other granu- 

 lar masses widely separated fi'om each other, but occupying the 

 posterior half of the embryo. These Leuckart supposes to be the 

 rudiments of a fixture brood to be developed at the time when the fi:-ee 

 embryo shall have lost its ciliated swimming apparatus, shall have 

 bored its way by means of the cephahc spine into the tissues of a 

 mollusc, and shall have become metamorphosed into a sac-like 



