262 



ZOOLOGY 



sitic in Fishes the head bears four prominent thin folded flaps — 

 the hothridia, which are exceedingly mobile, and are used more as 

 creeping organs than as organs of fixation. In ]-elation to each of 

 these bothridia, which, by coalescence, may appear to be reduced 

 to two, may be a small sucker of the ordinary kind. In Tetra- 

 rhynchus (Fig. 203) there are four very long and narrow rostella, 

 or " proboscides," covered with booklets, and capable of being 

 retracted into sheaths. 



The Cestoda are devoid of mouth, and in most of them the 

 genital apertures are marginally placed, so that, externally, there 

 is — except in the case of a few in which the genital apertures are 

 not marginal — nothing to distinguish the dorsal surface from the 

 ventral. The lody, or strohila, which is narrower in front than it 

 becomes further back, is made up throughout its length of a series 

 of segments, or proglottides, which become larger and more dis- 

 tinctly marked off from one another as we pass backwards. Tcenia 

 echinococcus (Fig. 204) is exceptional in possessing only three or 

 four proglottides. In a few {Ligula and its allies — Fig. 205), 



though the body has the normal elongated ribbon-like form, the 

 segments are not distinct, and in CaryophyllcGus (Fig. 206), 

 Am^ihilina, Gyrocotyle {Ampliiptyclies — Fig. 207), and Archigetes 

 (Fig. 208) — (Monozoa), segmentation is entirely absent, the whole 

 body in 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 Platyhelminthes 

 in general there are integumentary layers and underlying layers of 

 muscle, whicli are more highly differentiated than in the Coelen- 

 terates. But considerable differences exist in this respect between 

 the members of the three classes. In the Turbellaria (Fig. 209) 

 there is, as already noticed in the account given of the tlanarian, 

 a distinct epidermis (cp>.) in the form of a layer of cells, most of 

 which are ciliated. A delicate cuticle is usually, though not always, 

 distinguishable, investing the epidermis externally. In one family 

 the cuticle is developed, along the margin of the body, into a series 



