784 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Adaptation to Environment in the Trematoda.* — Prof. G. B. 



Ercolani finds that : — 



1. The succession of phases of development is not always the same 

 in all Trematoda ; some leave the egg as a ciliated embryo, and require 

 water; others, developed in terrestrial molluscs, have a non-ciliated 

 embryo. 



2. Nor are the different phases in development the same for all ; 

 the condition of encystation which is necessary for some is omitted in 

 other species, which pass directly from the free cercaria into the free 

 Distomum. There is, moreover, at least one exception to the rule that 

 the larva must at one stage be agamic. 



3. The well-known fact that certain nurses are reproduced by 

 asexual generation (either gemmiparous or scissiparous, and the 

 latter either endogenous or exogenous) was observed not only in 

 simple sporocysts, but also in true Bedice. A special form of scissi- 

 parous generation was observed in the racemose sporocysts, where 

 certain living parts are (as in Bryozoa and Hydrozoa) connected by 

 atrophied stolons. 



4. The direct conversion of the tail of a cercaria into a nurse was 

 observed several times. 



5. Encystation may not only be normal, but also accidental or 

 abnormal ; some die when, and at the place where, this accidental 

 encystation takes place ; others become, sometimes completely, but 

 more frequently incompletely, adapted to this modification ; in the 

 latter case the generative organs are imperfectly developed, or are not 

 developed at all. Examples of this are to be seen in the adaptation 

 of Cercaria ecJiinula to the intestine of the duck, dog, or rat ; this 

 species accommodates itself in different ways, so as to present different 

 zoological characters, though these are not sufficiently distinguished 

 one from another to justify the formation of distinct species. On the 

 other hand, Distomum mentulatum may present quite definite specific 

 differences. 



The doctrine that each species of mollusc has a single determinate 

 species of cercaria, corresponding to a single species of Trematode, is 

 denied, and it is shown that, e. g. Bytinia tentaculata has as many 

 as twelve different species of cercarise. When exogenous gemmation 

 obtains, the buds are produced at the hinder end of the body. While 

 some forms have an excretory apparatus, composed of two vessels con- 

 verging towards a buccal pore, others have no vessels or pores. As 

 to the number of Distomata in one cyst, we had no definite information 

 prior to the observation of Ercolani that from 20-80 larva? might be 

 found in large cysts on the peritoneum of tadpoles. 



Vascular Organs of Trematoda.f — A. Villot points out that in 

 the Trematoda, as in the Cestoda, there are a large number of canals 

 which traverse the whole of the body, and open by a number of 

 pores, either on the surface or into the intestine. Although they con- 

 stitute but a single system, they may be divided into (1) a central 



* Arch. Ital. Biol., i. (18S2) pp. 439-53. 

 f Zool. Anzeig., v. (1882) pp. 505-8. 



