FLAT-WORMS. 



465 



organ is supported by a set of radiating spines, eight in number, and has a soft 

 membranaceous rim. The head is surrounded by a circlet of small feelers. This 

 worm — one of the smallest of its group — lives as an external parasite upon annelids, 

 especially upon tube-making forms, such as Clymene. 



The worms which constitute a second section of the present suborder differ 

 from the foregoing in possessing several sucking-discs at the hinder end of the 

 body. Among them is a curious creature well deserving its name of Diplozoum 



life-history of double-worm (magnified). 



paradoxum, since it consists of two complete, mature similar halves, each possessing 

 every attribute of a perfect animal (a). Each of the pointed front ends has a mouth 

 aperture, and close to it a couple of small sucking-discs; while each individual 

 has a separate intestine consisting of a median tube and innumerable side-branches. 

 At the hinder end of the body are two suckers sunk in a depression, and protected 

 by four hard buckle-shaped organs. The double- worm lives on the gills of several 

 species of fresh-water fish, the gudgeon and minnow for instance. The eggs are 

 elongate and provided at one end with a fine thread-like appendage (6). In this 

 egg the young (c) — which at the time of hatching is only about one hundredth of 

 an inch — takes about a fortnight to develop. It is covered with cilia, has two 

 vol. vi. — 30 



