172 Introduction to Animal Morphology. 



CHAPTER XXV. 



ORDER 2. — TREMATODA [Rudolphi). 



Flattened, lanceolate, or oval, mono-metameral, 

 endo- or ecto-parasites, with one or more ventral 

 suckers, and two pharyngeal nerve ganglia united 

 by a flat commissure, sending off weak lateral 

 nerves with no neurilemma, a branch to the oral 

 sucker and one to the digestive organs ; a ganglion 

 sometimes underlies the ventral sucker (Distoma 

 lanceolatum). The cuticle is armed with bristles, 

 spines, or scales, which constitute a head armature 

 in D. echinatum, militare. The dermis is cellular 

 or protoplasmic, over a strong trilaminar muscular 

 stratum, whose inner, circular lamina mostly consists 

 of spindle-cells, and a connective parenchyma making 

 up the chief bulk. The suckers have strong, radiating, 

 and weaker circular fibres, and in the centre of the 

 anterior one (except in Polystomidae) is the mouth, 

 leading into the muscular, suctorial, often globular 

 pharynx. A narrow oesophagus joins this to the in- 

 testine, which is usually an excavation in the paren- 

 chyma, lined by cylinder epithelium, and with no 

 muscular coat ;* it is rarely simple (Aspidogaster, 

 Gasterostomum) ; usually forked or branched, aproc- 

 tous ; the branches are usually caecal, rarely anasto- 

 mosing ;t some, perhaps, hepatic. In D. filicolle, the 

 intestine disappears in the adult. 



* A muscular coat is described by Leuckart. 



t The two limbs of the intestine may unite to form a ring (Gynoe- 

 cophorus), which gives off branches (Epibdella, &c.), sometimes giving off 

 a central azygous process (Polystomum integerrimum) . 



