ANATOMY OF POLYCLADIDA 



pelagic existence till they are three or four millimetres in length. 

 In late summer, numbers of such immature examples may be found 

 among sea-weeds and Corallina in tide pools. In the succeeding 

 spring they develop first the male and then the female reproduc- 

 tive organs. 



Dorsal 

 ovd par ;h _cil 



rtwi 



r.d 



Ventral 



FIG. 4. Portion of a transverse section of Leptoplana tremdlaris in the hinder part of 

 the body, x 100. bm, Basement (skeletal) membrane ; cil, cilia ; d.m, diagonal 

 muscles ; d.v.m, dorso-ventral muscles ; ep, epidermis ; f.p, food particles : l.g, lateral 

 intestinal branches cut across ; l.m ext, external, and l.m int, internal longitudinal 

 muscle layers ; m.c, glandular (mucous) cells ; md, their ducts ; N, longitudinal 

 nerve ; Nu, nuclei of the intestinal epithelium ; ov, ovary ; ovd, oviduct ; par, 

 cells of the parenchyma ; r.d, vasa deferentia, with spermatozoa ; rm, circular 

 musculature ; rh, rhabdites ; sh, cells of the shell-gland ; te, testes ; ve, vasa 

 efferentia ; y.c, "yellow cells." (After Lang.) 



Anatomy of Leptoplana tremellaris. Leptoplana may be 

 divided into corresponding halves only by a median vertical 

 longitudinal plane. The body and all the systems of organs are 

 strictly bilaterally symmetrical. Excepting the cavities of tl i 

 organs themselves, the body is solid. A connective " parenchyma " 

 (Fig. 4, par} knits the various internal organs together, while it 

 allows free play of one part on another. These organs are enclosed 

 in a muscular body-wall, clothed externally by the ciliated epidermis, 

 which is separated from the underlying musculature by a strong 

 membrane (Fig. 4, lin), the only skeletal element in the body. 



