152 



COMPARATIVE ANATOMY 



CHAP. 



finely lacunar by the formation of many vacuoles filled with fluid. 

 The lacunse may coalesce in sinuses which conduct fluid, which, however, 

 generally remain small ; but in a few Rhabdoccela they become large 

 hollow spaces filled with a perivisceral fluid. In such cases the body 

 parenchyma can assume the constitution of a membrane, covering the 

 inner organs like an epithelium. In the Accela, where there is no 

 parenchyma separate from the intestine, the former, composed of star- 

 shaped cells filling the whole body apart from the specific organs, may 

 be described as digesting parenchyma. 



XL The Excretory or Water-vascular System. 

 This is very characteristic of the Platocles, and as yet has been 



found to be Avanting only in the Acceta among the Rhaldoccelidct 

 consists of a system of very fine transparent 

 canals (excretory capillaries) which branch out 

 in the parenchyma and between the muscles, 

 and which enter into a system of wider, equally 

 transparent canals, which open externally in 

 various ways. In the formation of the ex- 

 tremely thin walls of the capillaries only a few 

 cells take part, so that the nuclei lying in the 

 wall occur at long intervals, and in a trans- 

 verse section of a canal the wall enclosing the 

 central lumen belongs to a single cell. The 

 capillaries thus represent perforations of linear 

 rows of cells, and are described as intracellular. 

 The wider canals, on the other hand, in the 



It 



FIG. 109. Excretory cell 

 at the end of a fine excre- 

 tory canal (t) of a Turbel- 

 . . -. ^ . i larian. n, Nucleus ; v, vacu- 



Cestoda at any rate, appear lined by a thin epi- ole . fj proce g 8es O f the ceil ; 

 thelium, and are thus intercellular. The blind ?/, flame. 

 end of each excretory capillary is formed by one 



cell (Fig. 109), which possesses fine protoplasmic processes running 

 into the parenchyma. In this cell, which, on the surface turned 

 to the lumen of the capillary, carries a tuft of fine vibrating cilia 

 (the flame) projecting into the lumen, excretory products (drops, 

 granules, etc.) collect and are emptied out of the cell into the 

 capillary. The excretory products are forwarded out of the capil- 

 laries into the wider vessels partly by the motion of the above- 

 mentioned cilia, and partly perhaps by the independent contractions of 

 the canals, and thence reach the exterior. Sometimes also in the 

 lateral walls of the capillaries and larger canals flames are found which 

 belong to the excretory cells in those walls. The larger canals some- 

 times have a continuous lining of cilia. It is not impossible that the 

 greater part of the transparent fluid which fills the canals is water 

 taken in from outside. If so, it is occasionally emptied out and again 

 taken in. In this way the water-vascular system may also perform 

 a respiratory function. 



