724 OCCURRENCE AND DEVELOPMENT OF BOTHRIOCEPHALUS LATUS. 



mantle are squeezed through the narrow opening, and the play of the 

 ciliated hairs begins, if it have not previously begun in the interior of 

 the egg. The liberated embryo immediately resumes its round form, 

 and swims about in the water by means of its cilia, rotating continu- 

 ally on its axis, with the hooks directed backwards. The cilia are 

 extremely thin and delicate, and can only be recognised to their full 

 extent in a good light. They are disposed very close to each other, 

 and in contradistinction to the other species (Fig. 386), and also to 

 Knoch's description, are of so considerable a length that they exceed 

 the diameter of the embryonal body. In the interior of the egg-shell 

 one usually sees, besides the remains of the yolk-cells, the nucleated 

 enveloping membrane. 



The period of incubation is extremely variable, being determined 

 by the surrounding temperature and the depth of the water. In 

 the middle of summer, when the eggs had been kept in flat saucers, I 

 have often seen the embryos escape before the end of a month, and 

 in the incubator, with a temperature of 37'5 C., after fourteen days. 

 Under other circumstances, several months, or if the winter intervene, 

 many months (eight or more), may elapse before the embryos are 

 hatched, even when the extent of their development suggests that they 

 should have come out some time previously. 



In the free state the embryos continue to live and move for some 

 days, sometimes more than a week. By means of their cilia they 

 swim constantly, but somewhat slowly and heavily, through the water. 

 The form of the body is but little altered, though sometimes rather 

 elongated and sometimes shortened, so that the poles become flat- 

 tened, or there may even be formed a slight pit or depression. This 

 is especially true of the anterior bookless pole, which during strong 

 contraction is broader than the posterior. After a short time the 

 two lamellae of the mantle separate considerably from each other owing 

 to the inception of water. The mass enclosed between them becomes 

 consequently clearer and more transparent, and the nuclei, formerly 

 so numerous, disappear. In place of these, however, it now appears 

 (Fig. 386, B) as though the space between the lamellae were occupied 

 by clear vesicular spaces, closely arranged in a single layer, so that they 

 flatten out almost prismatically at the points of contact, and squeeze 

 between them the granules which were originally more equally dis- 

 tributed throughout the whole mass. I have previously (p. 329) inter- 

 preted these vesicles without hesitation as cells, as Bertolus had also 

 done, but now I find myself compelled to modify my opinion, and to 

 regard them as vacuoles, which have probably arisen simply in conse- 

 quence of the absorption of water. In this interpretation I differ on 

 the whole but little from Schauinsland, who refers this appearance of 



