248 



ZOOLOGY 



SECT. 



these genera consisting of a single proglottis. The surface in the 

 Cestodes is devoid of cilia, and there is no pigment. 



Integument and Muscular Layers. In the Platyhel- 

 minthes in general there are integumentary layers and underlying 

 layers of musck, which are more highly differentiated than in the 

 Coslenterates. But considerable differences exist between the 

 members of the three classes. In the Turbellaria (Fig. 198) there is, 

 as already noticed in the account given of the Planarian.a distinct 

 epidermis (Fig. 198, ep) in the form of a layer of cells, most of 



which are ciliated. A deli- 

 cate cuticle is usually, though 

 not always, distinguishable, 

 investing 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 epi- 

 dermal cells there are in the 

 Polycladida numerous cells 

 containing short rod-like 

 bodies the rltaldites (rk.) ; 

 in the other orders of the 

 Turbellaria these rhabdite- 

 forming cells are sunk deeply 



d.vrn. 



rh. c 



FIG. 198. Section of the body-wall of u Triclad. 

 b. -in. basement membrane; ?. /. circular 

 muscles ; (/. -?. /,/. dorso-ventral muscles ; 

 e. I. m. external longitudinal muscles; < />. 

 epidermis ; i. I. M. internal longitudinal 

 muscles; p. parenchyma; rh. rhabdites; 

 rk. c. rhabdite-formiug cells. (After Jijirua.) 



within the parenchyma, and, 

 in the Rhabdoccela, have very 

 long ducts, formed of pro- 

 cesses 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 extremity. The func- 

 tion of these rhabdites is not in all cases certain ; 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 Rhabdocoela and Tricladida they un- 

 doubtedly aid in adhesion, and probably have the function of 

 assisting in the entanglement and capture of food. In certain of 

 the Turbellaria stinging capsules occur similar to those of the 

 Coslenterata, and transition forms between rhabdites and stinging 

 capsules occur in some cases. Adhesive cells, with processes also 

 frequently occur in the epidermis. Beneath the epidermis is a 

 basement membrane (6. m.\ which in the Polycladida is of a thick 

 resistent character, and contains stellate cells. 



_ In a small number of the Trematoda three distinct layers are 

 distinguishable in the integument a homogeneous, or nearly 



