2l8 THE INVERTEBRATA 



ectoderm is represented by cells sunk into the parenchyma in much 

 the same way as nuclei of the ectodermal cells in the pharynx of the 

 Tricladida. But the outer portion of the cell is lost in the Trematoda 

 and its place is taken by a thick cuticle, which is often armed with 

 spines. Suckers are always present for attachment to the host and 

 are of large size. The presence of these suckers and their shape makes 

 it possible to divide the Trematoda proper into two orders : (i) Hetero- 

 cotylea, (ii) Malacocotylea. 



Order HETEROCOTYLEA 

 In the Heterocotylea there is a large posterior sucker stiffened with 

 chitinous supports. It is often subdivided, as in Octohothrium or 

 Polystomutn (Fig. 158). In the Malacocotylea the sucker is not always 

 posterior, it often moves forward on the ventral surface so that, as in 

 Fasciola, it comes to lie one-third of the body-length from the anterior 

 end. It is never provided with chitinous supports. All the Hetero- 

 cotylea are ectoparasites with the single exception of Po/y^^owwm which 

 occurs in the bladder of the common frog, of which from 3 to 10 per 

 cent, are infected by it. They are confined to one host only. The 

 Malacocotylea are all internal parasites and pass from one host to 

 another at certain stages in their life history. In the Heterocotylea the 

 excretory pores are paired and lie near the anterior end of the body, 

 whereas in the Malacocotylea the excretory system discharges to the 

 exterior through a single median pore placed at the posterior end of 

 the body. In the Heterocotylea there are separate openings for the 

 male and female portions of the generative system, while in the 

 Malacocotylea there is but one common opening. In the Heterocotylea 

 there is a pair of ducts leading from the ootype to the exterior indepen- 

 dently from the male and female ducts, usually called the vaginae. 

 The vaginae are inconspicuous as a rule, but in Polystomutn their 

 openings are very clearly marked by two prominences on either side 

 of the body about one-fifth of the body-length from the anterior end 

 (Fig. 158). Corresponding ducts do not occur in the Malacocotylea. 

 The nervous system of the Heterocotylea is more primitive than that 

 of the Malacocotylea, but in both groups it is stereotyped and does 

 not vary as it does in the Turbellaria. In both groups it consists of a 

 cerebral ganglion with six cords leading posteriorly. In the Hetero- 

 cotylea there are irregular commissures between the cords, while in 

 the Malacocotylea the commissures are few in number and regular. 

 Life history of the Heterocotylea. The usual habitat of this order is 

 on the gills of fishes where they often live isolated. Self-fertilization 

 must therefore be practised, but copulation has been observed in 

 Polystomutn and also in Diplozoon, where it is permanent. The 

 members of this order probably cause considerable inconvenience 



