310 



PLA.TTHELMINTHES. 



alimentary canal. Hooks and suckers are absent. A cerebral ganglion 

 is present. 



The Turbellaria usually possess an oval flattened body, and reach 

 only a small size. The uniform ciliation of the body is connected 

 with their existence in fresh and salt water, beneath stones, in mud, 

 and even in damp earth. Only in exceptional cases do we meet 

 with apparatuses for adhering, viz., small hooks and suckers. 



The skin consists of a single layer of cells, or of a finely granular 



layer containing nuclei, which is sup- 

 ported by a stratified basal membrane, 

 and covered externally by a special 

 homogeneous membrane bearing cilia 

 and comparable to a cuticula. Peculiar 

 integumentary structures, which have 

 the form of rods or spindles, and, like 

 the nematocysts in Coelenterata, take 

 their origin, in cells, are not unfre- 

 quently present. Various pigments are 

 also often found embedded in the epi- 

 dermis, and of these pigments the green- 

 coloured vesicles, in Vortex viridis for 

 example, which are identical with chlo- 

 rophyl corpuscles, are specially worthy 

 of remark. Pear-shaped mucous glands 

 are also present. Beneath the conspicu- 

 ous basement membrane which supports 

 the epidermis lies the dermis. It con- 

 tains the strongly developed derma] 

 muscular system embedded in a connec- 

 tive tissue layer formed of round, often 



branched cells. A body cavity between 

 FIG. 247. Alimentary canal and ner- , , , n -, ,. 



vous system of af->rfwa &,*. the bodv wal1 and the alimentary canal, 



rgii (after Graff). G, the two i s? as a ru i e) absent : it may, however, 



cerebral ganglia with two eye 



spots; st, the two lateral nerve in many cases be recognised as a system 



trunks; L '.alimentary canal with of lacunse or as a continuous cavity 

 mouth and pharynx. 



surrounding the alimentary canal. 



The nervous system consists of two ganglia connected by a com- 

 missure, and giving off nerve fibres in various directions ; of these, 

 two especially large lateral trunks run backwards, one on either side 

 (fig. 247). The latter are connected at regular intervals by delicate 

 transverse trunks. In a number of dendrocoelous Turbellarians a 



