162 THE ANATOMY OF INVERTEBRATED ANIMALS. 



canals of the ordinary kind ; but in the land Planarians ^ two 

 nearly simple canals, occupied by a spongy tissue, and the 

 connection of which with the exterior has not been observed, 

 occupy the place of the water- vessels. 



Tlie fresh-water Plaiiarim^ like the Rhahdoccela^ undergo 

 no metamorphosis in the course of their development ; and 

 the like is true of some of the marine Dendrocmla. Kefer- 

 stein^ has carefully worked out the development of Lepto- 

 plana [Poly cells). The vitellus undergoes division first into 

 two and then into four equal blastomeres ; next, from one 

 surface of these four blastomeres, four small segments are, as 

 it were, pinched off. These divide rapidly, and form a blas- 

 toderm, which grows over the more slowly dividing large seg- 

 ments, and eventually incloses them. So far, the process is 

 verv similar to that which has been described in the Cteno- 

 pliora. But though Keferstein describes and figures the 

 various stages by which the globular ciliated embrj^o attains 

 the form of the adult, neither his description nor the figures 

 enable one to say whether the alimentary cavity arises by de- 

 lamination or by invagination, nor to trace the mode of origi- 

 nation of the buccal proboscisough, th this organ is one of 

 the first to make its appearance, and its aperture becomes the 

 future mouth. 



In some of the marine PkmaricB, hovrever, the embryo, 

 when it leaves the egg, differs very widely from the adult. 

 Johannes Mliller described such a larva, in which the body is 

 provided with eight lobes or processes, one ventral and median 

 in front of the mouth, three lateral, and one dorso-median. 

 The edges of these processes are fringed by a continuous 

 series of cilia, which pass fron one process on to another, so 

 as to form a complete circlet round the bod}^ The successive 

 working of the cilia forming this lobed transverse girdle of the 

 body produces the appearance of a rotating wheel, as in the 

 Poiifera. The eyes are situated on the aboral face of the 

 embryo, in front of the ciliated circlet, while the mouth opens 

 immediately behind it. As development proceeds, the lobes 

 disappear, and the body takes on the ordinary Planarian 

 character. 



As will be seen, some of the Proctucha have larvae simi- 

 larly provided with a ]Dra3-oral ciliated zone ; and laivffi of 



* Moselev, " On the Anatomv and Histology of the Land Planarians of Cey- 

 lon." (" Philosophical Transactions," 1873.) " 



'^ "Beitrage zur Auatomie uud Entwickelungsgeschichte ciniger See-Plana- 

 rien ," 1868. 



