180 THE ANATOMY OF INVERTEBRATED ANIMALS. 



with long tails, by which they are propelled. These creatures, 

 called Cercarioe (Fig. 44, E), escape by bursting through the 

 Redia^ and, after a free-swimming existence, penetrate the 

 body of some other animal, their tails dropping off. They 

 then become encysted, and, under suitable conditions, assume 

 the adult form, and develop reproductive organs (Fig. 44, F). 

 The cycle of forms through which Distoma mllitare passes 

 has been nearly completely traced, and may be briefly stated 

 as follows : 1. The parent form, whose habitat is the in- 

 testines of water-birds, bears on its anterior extremity two 

 alternating circles of larger and smaller booklets, and a few 

 others, irregularly disposed. Rings of papillae give the cen- 

 tre of the body an annulated aspect. The mouth, almost 

 terminal, leads into the long, straight digestive c?ecum. The 

 generative organs are similar to those of Aspidogaster ; the 

 testes are, however, double, and lack the internal vas deferens. 

 The ova are few, eight or ten in number. 2. From each 

 ovum issues a ciliated larva, showing the rudiments of — 3. A 

 Redia^ but the mode of development of the latter has not 

 been fully traced. The perfect Media is found attached to 

 the body of a water-snail [Paludlna)^ the ciliated investment 

 having disappeared. It consists of a sac, within which is 

 suspended a tubular bag, containing colored masses, probably 

 alimentarv. Anteriorly, the head is represented by a kind of 

 crown, in which no oesophagus exists as yet, and not far from 

 the posterior extremity the two lateral projections, character- 

 istic of Distomatous Redke, appear. During the rapid growth 

 of the zooid, the head becomes marked off by a constriction, 

 and a mouth and gullet, with a pharyngeal dilatation, admit 

 aliment to the digestive sac. In the body cavit}^, external to 

 this sac, vesicles appear, rapidly increase, and take the form 

 of Cercarim ; the Redla bursts, and these new zooids are 

 set free. 4. The Cercaria has a long tail with lateral mem- 

 branous expansions, by means of which it swims after the 

 fashion of a tadpole. The pharyngeal bulb is followed by an 

 oesophagus, which, opposite the ventral sucker, divides ; the 

 two branches ending in a caecum on either side of the con- 

 tractile vacuoles of the water-vascular sj'stem. These are 

 median, the terminal quadrate chamber opening into an an- 

 terior circular one, whence are given off the two main canals 

 which traverse the body longitudinally, and are then lost. 5. 

 After swimming about freely for a while, the Cercaria fixes 

 itself upon, or bores its way into, a Paludina ; the tail drop- 

 ping off, and the body coating itself with a structureless cyst, 



