PHYLUM PLATYHELMINTHES 



257 



Integument and Muscular Layers. In the Platyhelminthes 

 in general there are integumentary layers and underlying layers of 

 muscle, which are more 

 highly differentiated than 

 in the Coelenterates. But 

 considerable differences exist 

 in this respect between the 

 members of the three classes. 

 In the Turbellaria (Fig. 212) 

 there is. as already noticed 

 in the account given of the 

 Planarian, a distinct epider- 

 mis (ep.) in the form of a 

 layer of cells, most of which 

 are ciliated. A delicate 

 cuticle is usually, though not 

 always, distinguishable, in- 

 vesting the epidermis ex- 

 ternally. In one family the 

 cuticle is developed, along 

 the margin of the body, into 

 a series of chitinous bristles. 

 Among the ordinary epider- 

 mal cells there are in the 

 Polycladida numerous cells 

 containing short rod-like 



s _ 4-hp rhahdiffxi (rh \ FlG - 206. Tetrarhynchus. Fio. 207. Tsenia 



IBS \rn.) , n nervous system' r. pro- echinococcus. 



in the Other Orders Of the boscides ; rs. sheaths, with (After Cobbold.) 



m i 11 .1 T. -L TJ. their muscles (rb.). (From 



Turbellaria these rhabdlte- Leuckart, after Pintner.) 



forming cells are sunk 



deeply within the parenchyma, and, in the Rhabdocoela, have 

 very long ducts, formed of processes of the cells, by means of 

 which the rods, together with a viscid matter, reach the exterior 



at certain points of the surface 

 chiefly around the anterior ex- 

 tremity. The function of these 

 rhabdites is not in all cases cer- 

 tain ; they have been supposed 

 to add to the sensitiveness of 

 the parts in which they are 

 situated after the fashion of 

 hairs or nails, or to have a 

 skeletal function. In the Rhab- 



FIO. 208.-i.iguia. (After Leuckart.) doccela and Tricladida they 



undoubtedly aid in adhesion, and 



probably have the function of assisting in the entanglement 

 and capture of food. In certain of the Turbellaria stinging 



VOL. i. s 



