642 THE ANIMAL KINGDOM. 



CLASS TREMATODA. 



Unisegmental Vermes, with a flattish leaflike or more or less cylind- 

 rical body provided with organs of adhesion in the shape of suckers, and 

 sometimes of chitinoid hooks. The cuticle so-called appears to be a meta- 

 morphosed layer of cells. There is a well-developed nervous system, the 

 ganglia of which are entirely supra-pharyngeal, i. e. dorsal. There is a 

 mouth, and an alimentary canal which is usually forked, biit no anus. The 

 excretory system has the form of more or less branching tubes commencing 

 with flame-cells, and either ending in a contractile vesicle, or opening by two 

 independent orifices. Hermaphrodite. Self -impregnation occurs, as well as 

 reciprocal impregnation. The embryo either developes direct into the sexual 

 form monogenetic Trematoda, or gives origin to a series of intermediate 

 non-sexual dimorphic forms digenetic Trematoda. Parasitic. 



The monogenetic Trematoda are either roundish or elongated in 

 shape. Two small oral suckers are often present, one on either side 

 the mouth, while the posterior end of the body is either provided with 

 a single sucker, usually large and sometimes armed with chitinoid hooks, 

 or it is expanded and furnished with suckers, or hooks, or with both hooks 

 and suckers or claspers. The digenetic Trematoda are usually elongate, 

 and either flattish or rounded. They have an oral sucker, and there is, 

 except in Monostomidae, a second, usually ventral, sometimes posterior as 

 in Amphistomum, but anterior in G aster ostomtim where the mouth is 

 sub-median and ventral. A large number of suckers is rare. The suckers 

 themselves are usually cup-shaped, sometimes compartmented as in 

 the large ventral sucker of Tristomum and in Aspidogaster. They are 

 either sessile or stalked. When numerous they may be set close together 

 in a series of rows on the ventral aspect, e. g. Gastrodiscus from the horse ; 

 in three dorsal ( ? ventral) rows as in Notocotyle ( = Monostomum verru- 

 cosum) from the duck, coot, &c. ; in a single ventral row as in Stichocotyle 

 Nephropis found encysted in the Crustacean Nephrops, or in a row on 

 the margins of the body as in some monogenetic Trematoda. The 

 chitinoid hooks of the group just named are very generally situated on the 

 suckers. The monogenetic family Gyrodactylidae is remarkable for 

 possessing either 2-4 retractile adoral processes, or an incomplete circum- 

 oral lobe (Calceostoma) as well as one or two posterior terminal discs armed 

 with hooks (= retinacula). The claspers found in some Polystomeae are 

 provided with two muscular valves supported by chitinoid bars : one valve 

 is fixed, the other free : and the free valve has a snapping action. These 

 last named structures have been accurately investigated only in Axine 

 and Microcotyle. 



There is a cuticle, variable in thickness, sometimes perforated by pores, 



