ii TREMATODA MONOGENEA 53 



Trematoda monogenea (cctoparasitica). 



There are four subdivisions of the Monogenea : 



I. Temnocephalidae, with four to twelve tentacles, and one 

 sucker posteriorly (Fig. 20). 



II. Tristomatidae, with two lateral, anteriorly-placed suckers. 

 Oral suckers are absent, a large posterior sucker is constant, and 

 is often armed with hooks (Fig. 22, C). 



III. Polystomatidae, with, usually, two oral suckers and a 

 posteriorly-placed adhesive disc armed with suckers and hooks 

 (Figs. 23 and 24). 



IV. Gyrodactylidae (Fig. 29). 



Habits and Structure of Ectoparasitic Trematodes. 



I. Temnocephalidae. These interesting forms, of which a good 

 account has lately been written by Haswell, 1 occur on the surface 

 (rarely in the branchial chamber) of fresh-water crayfish and 

 crabs in Australasia, the Malay Archipelago, Madagascar, and 

 Chili. Others have been found on the carapace of a fresh-water 

 tortoise, and in the branchial chamber of the mollusc Ampvllaria 

 from Brazil. Wood-Mason discovered others, again, in bottles 

 containing spirit -specimens of Indian fish. Temnocephala is 

 rarely more than a quarter of an inch long, and looks like a 

 minute Cephalopod or a broad flattened Hydra. By the ventral 

 sucker each species adheres to its own particular host, the ten- 

 tacles being used as an anterior sucker for " looping " move- 

 ments. The food, consisting of Entomostraca, Kotifera, and 

 Diatoms, is first swallowed whole by the large pharynx (Fig. 20, 

 ph), which can be protruded through the ventrally-placed mouth, 

 and is then received into a simple lobed intestine (d). The skin, 

 especially on the surface of the tentacles, is provided here and 

 there with patches of cilia borne by the cellular epidermis, 

 the only undoubted case of external cilia occurring in an adult 

 Trematode. Minute rhabdites formed in special gland -cells, 

 occur plentifully on the tentacles, and are another distinctly 

 Turbellarian feature. The excretory system is peculiar (Fig. 21). 

 Fine ducts proceed from the various organs of the body, and 

 open to the exterior by means of a pair of contractile sacs 



1 Haswell, Monograph of (he Temnocephaleac, Macleay Memorial Volume. Mem. 

 iii. 1893. 



