TREMATODA. 651 



Molluscs especially may be liable to infection. After its entry it becomes 

 in some instances a Sporocyst^ in others a Redia, which must be regarded 

 as dimorphic forms, and are sometimes spoken of as ' nurses.' Both pos- 

 sess a cuticle, a delicate layer of circular and longitudinal muscle fibres, and 

 a layer of cells covering the latter internally and bounding a cavity some- 

 times crossed by trabeculae of connective tissue in which lie developing 

 germs in all stages l . Flame-cells have been detected in the body-walls. 

 A Redia also possesses a digestive tract, i. e. a muscular pharynx (? = oral 

 sucker 2 ) and a caecal intestine, as well as a special opening for the birth of 

 the germs rarely present in Sporocysts. It has also two short processes, 

 one on either side of the posterior extremity of the body, as well as a pro- 

 jecting ring near its anterior extremity 3 . The Sporocyst possesses the 

 power, at least in some instances, of multiplying either by transverse fission, 

 as e.g. in F. hepatica, and the Cercariaeum Limacis not uncommon in the 

 slug Arion ater ; or by gemmation, producing branched structures, as in the 

 genera known as Bucephalus (= G aster ostomum) inhabiting A nodonta and 

 the Oyster, and Leucochloridium (= D. macrostomum) inhabiting the Gas- 

 tropod Succinea amphibia. The germs to which a Sporocyst gives origin 

 may develope in some instances into Sporocysts, in others into Rediae or 

 into Cercariae ; those which originate from a Redia may develope into 

 either Rediae or into Cercariae. And it does not seem certain that there is 

 any limit to the possible number of successive generations of Rediae. Both 

 Cercariae and Rediae may occur side by side in the same nurse. The last 

 term in the series is, however, invariably a Cercaria. The germs which 

 give origin to these various generations arise from two sources : the first, 

 cells which occupy the central region of the young Sporocyst or Redia ; the 

 second, the epithelium lining the body-walls, single cells of which enlarge, 

 divide, form morulae, and drop into the cavity of the body to undergo 

 further development 4 . An invagination without a lumen to form a 

 digestive canal has been observed in the germs of F. hepatica ; and a 

 delicate pellicle has been seen surrounding the germ and young Cercaria. 



1 The Sporocyst may become enveloped in an adventitious epithelial coat or ' paletot,' formed 

 from the blood corpuscles of its host (Biehringer). 



3 A rudimentary oral sucker is present in some Sporocysts (Biehringer). 



3 For the structure of this ring and the mistakes that older observers have fallen into respecting 

 it, see Thomas, Q. J. M. xxiii. pp. 121-2, and note. 



* Leuckart restricts (A. N. 48, i, 18^2, p. 95, p. 100) the origin of germs to cells with large 

 nuclei (germinal cells) occupying the central region of the ' nurse. 5 The cells in question have no 

 share in the growth of the nurse itself. His view appears to be adopted by Schwarze. But as 

 Biehringer points out (i) there are great differences of size in the developing germs within the same 

 ' nurse,' too great to be explained by a temporary arrest of development of some of them ; and (a) it 

 is scarcely possible to extend the view to such cases as the branched Sporocyst of Bucephalus, and 

 he might have added, to Leucochloridium and such Sporocysts as multiply by repeated fission, e. g. 

 Cercariaeum Limacis. The origin of germs from the epithelium lining the body-cavity has been 

 established beyond doubt by Biehringer himself in the Sporocysts of Cercaria macrocerca from the 

 gills of Cyclas, and by Thomas in the ' nurse ' forms of F. hepatica. 



