TREMATODA 213 



Suckers are always present and occur in varying numbers and 

 positions at the anterior extremities as well as on the ventral surface, 

 and occasionally on the lateral margin and on the dorsum ; the 

 beginning of the intestine (mouth) is always surrounded by a sucker 

 in the Prostomata. 



In or near the suckers there may be chitinous hooks, claws or 

 claspers, or the surface of the body is more or less covered with spines, 

 scales or prickles; in one genus (Rhopalias) there are projectile tentacles 

 beset with spines on the sides of the anterior part of the body. 



The body of adult Trematodes is covered by a homogeneous layer 

 of varying thickness, which either lies directly over the external 

 layer (basement membrane) of the parenchyma, or over the muscles 

 embedded in the parenchyma. This investing membrane (cuticle) 

 arises from pear-shaped or spindle-shaped cells arranged singly or in 

 groups (which lie between or internal to the diagonal muscles), and 

 is connected with them by processes ; these cells one may regard as 

 epithelial cells which have sunk down, or possibly as parenchymatous 

 cells. An epithelium of one layer is also found on the body of 

 young stages, but it disappears during growth, and only occasionally 

 do its nuclei persist until adult life. In its place we then find the 

 cuticle, which, moreover, extends into all the body openings more or 

 less deeply. 



It is thus a debatable point whether the " investing layer " of flukes 

 is a cuticle that is, consists of modified epithelial cells or whether it is 

 a basement membrane, i.e., compressed and modified connective tissue 

 cells ; in this latter case the true epidermis and cuticle have been cast 

 off. In the former case the epidermal cells are the pear-shaped cells 

 referred to above. According to recent authors it consists of two 

 parts, an outer true cuticle and an inner basement membrane. There 

 are also unicellular cuticular glands, lying isolated or in groups, which 

 are termed cephalic, abdominal, or dorsal glands according to the 

 position of their orifice. 



The PARENCHYMA is a connective substance, the structure of 

 which is still a matter of dispute. It consists, according to some 

 authors, of multipolar cells, the offshoots from which anastomose with 

 each other so that a network, permeating the entire body and encom- 

 passing all the organs, is produced. There exists also, as part of it, a 

 homogeneous matrix, in the form of lamellae and trabeculae that border 

 small cavities communicating with each other and filled with fluid. 

 According to other authors, the parenchyma of the Trematodes con- 

 sisted originally of cells, of which, however, only the cell membranes 

 remain, while the protoplasm has been liquefied except for small residua 

 around the nucleus. Between these cells an intercellular mass has 

 appeared. By partial absorption of the walls, adjoining spaces unite, 



