ANATOMY OF POLYCLADIDA 



I I 



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. 



mo- 

 md 



Ventral 



Fig. 4. Portion of a transverse section of Leptoplana tremellaris in the hinder part of 

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

 muscles ; cl.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 ; via, their ducts ; JV, longitudinal 

 nerve ; Nit, 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 the 

 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. hii), the only skeletal element in the body. 



