204 THE INVERTEBRATA 



Passing through the parenchyma and running dorsoventrally are 

 strands of muscle which are attached at either end to the dorsal and 

 the ventral muscle layers. The muscles themselves consist of fibres 

 formed of a homogeneous transparent material that shows no trace 

 of any structure. These fibres are produced by a special cell, the 

 myoblast, which is often to be seen lying alongside the fibre it has 

 produced. 



The outer covering of a platyhelminth differs according to the 

 group to which it belongs. In the Turbellaria the outer covering is 

 formed of ectodermal cells. These are usually large and flat, sometimes 

 with peculiar branched nuclei as in Mesostomum, or smaller and with 

 round nuclei as in the majority of forms. Externally the cells are 

 ciliated, the cilia being arranged in tracts over the surface of the body. 

 Inside the cells are seen a number of crystalline, rod-shaped bodies, 



e. 



gen.po. 



Fig. 150. Planaria, x about 4. From Shipley and MacBride. e. eye; 

 ci.pit, ciliated pits at side of head; M. mouth at end of protruded pharynx; 

 ph. outline of the pharynx sheath into which the pharynx can be with- 

 drawn ; gen.po. generative pore. 



known as rhabdites. Although much has been written about rhabdites 

 their function remains obscure. They are a secretion, more or less 

 firm, which dissolves and becomes liquid in contact with water. They 

 are formed in special cells, lying either between the ectoderm cells or 

 just beneath them in the parenchyma, and distributed thence ,to the 

 ectoderm cells. Rhabdites are usually absent from the ectoderm cells 

 in the neighbourhood of sense organs. It will be noticed that when 

 Turbellaria are placed for preservation in an irritant fluid such as 

 acetic acid the body becomes covered with an opaque white layer. 

 Whether this opaque layer is produced from the rhabdites or from 

 the slime glands which occur in certain regions of the body is not 

 certain. 



Immediately below the ectoderm lies the basement membrane. This 

 is a thin transparent structureless layer, which probably assists in 

 preserving the general shape of the body and serves as an attachment 

 for the muscles which lie immediately beneath it. 



