PLA TYHELMINTHES 



83 



contain a fluid in which certain cells float. Some of these 

 cells contain the yellow pigment which gives the animal its 

 yellowish colour. Unicellular slime - glands,- with longer or 

 shorter ducts, which open into the surface of the integument, 

 are comparatively numerous, and connective tissue cells are 

 also to be met with in the parenchyma. 



The rhdbdites are homogeneous refractive little rods 

 which occur in most Turbellaria, and are usually found in 

 great numbers either embedded in, or protruding from, the 

 epidermis. They escape from the body, and often occur in 

 great numbers in a sUmy deposit the animal leaves on its 

 track ; when they are expelled they leave a round hole in the 

 ectoderm cells. In some species they are secreted in the 

 ectoderm cells, in others, as in Mesostoma, from special cells 

 beneath the integument (Fig. 57); these cells, however, arise 

 from the ectoderm and remain connected with it by fine pro- 

 cesses along which the rhabdites travel. 



The connective tissue cells and the rhabdite-forming cells. 



Fig. 67. — Transverse section through the pharynx of Mesostoma Ehreribergiif 

 0. Sch. After Von Graff. 



1. Mouth. 



2. Pharynx walls crowded with glandular 



cells. 



3. Walls of alimentary canal. 



4. Salivary glands. 



5. Rhabdite-forming cells. 



6. Yolk glands. 



7. Testes. 



8. Nerve cord. 



9. Integument beneath cuticle. 



10. Pigment cell. 



11. Dorso-ventral muscle fibres. 



with some strands of muscle fibres, constitute the paren- 

 chyma. In Mesostoma this tissue is not so well developed as 

 in most Turbellaria, and the spaces containing fluid are corre- 



