572 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Bucephalus polymorphus, Cercaria acerca n. sp. (found in various 

 organs of Onchidium carpenteri), and another sporocyst from a species 

 of Onchidium from Singapore. 



In describing the structure of the sporocysts, he deals with the 

 epidermis, and points out that the so-called cuticle is not truly a cuticle, 

 but is a membrane in which a varying number of nuclei are to be detected; 

 its development is difficult to follow, but it would seem to be due to the 

 fusion by peripheral growth of some of the outer cells of the gastrula, 

 and to be comparable, therefore, to the eetoblast of the first order, 

 which has been described by Schauinsland in the embryos of Trema- 

 todes, and to the embryonic investment of Tcenice, as described by E. 

 van Beneden. On the whole, we are justified in regarding it as an 

 epidermis and comparing it with the " hypodermis " of other worms. 



The muscular layer is always very thin, and its outer layer consists 

 of delicate, closely applied, circular fibres ; below these is a longi- 

 tudinal layer, which is often much less distinct. Cercaria macro- 

 cerca is remarkable for having them broader and more distinct from 

 one another than they are in other forms. 



The germinal epithelium is in most cases unilaminate, and varies 

 in form in different species, the cells being cylindrical, cubical, or 

 flattened. C macrocerca is here again remarkable for having large 

 clear cells, which may be set in one or several layers. On their distal 

 side there are nuclei, which lie in a protoplasmic fundamental substance, 

 and which in section appears to form an anastomosing plexus. 



The so-called paletot is a fourth layer which is often present, and 

 which, in the opinion of Leuckart, is due not to the guest but to the 

 host ; and the author is of opinion that the substratum from which it 

 arises is the blood of the host, while the elements of which it is 

 composed are the cells of the snail's blood. After discussing this 

 question at some length Biehringer passes on to the sucker or depres- 

 sion which is often found at one pole of a sporocyst ; its structure 

 agrees so completely with that of the rest of the body-wall that it may 

 be considered as a mere invagination of the whole sac. It no doubt 

 serves as an organ of attachment. 



In dealing with the formation of the germinal bodies, and beginning 

 by discussing the views of previous writers, and especially of Leuckart 

 and A. P. Thomas, with the latter of whom he is in complete agree- 

 ment, he tells us that he is led by his own observations to think that 

 the developmental cycle of the Trematoda is a real case of alternation 

 of generation. 



In conclusion there are some remarks on the influence which the 

 gradually developing brood exercises on the organization and activity 

 of the sporocysts. When the brood remains at a lower grade of deve- 

 lopment the nurse contrives to grow; later on, when the daughter 

 generation is undergoing further development, it suffers a passive 

 extension, but this does not equally affect the whole of the body of 

 the nurse, but depends on the number and size of the germinal bodies 

 which are to be found in any given zone of its body. At last, the 

 whole mass forms a mere sac without any sign of organization, for 

 the brood at last completely destroys the body of the mother. 



