Dr. P. de Filippi on tlie Larvae of the Trematode Worms. 131 



ducts which terminate at the spine originate from a very pretty 

 bunch of secretory cells. This characteristic apparatus of the 

 armed Cercarise is not so well developed in any other species. 



Amongst the Cercarise which I have already made known in 

 my previous works, and which I have again frequently seen in 

 the course of this year, I shall mention particularly the Cercaria 

 virguloj of which I have indicated the analogy with the C. mia^o- 

 cohjla, parasitic on Paludina vivipara^. I must now add, that 

 in the form and dimensions of its Sporocysts, the C. virgula 

 presents the same differences that I have found in the other 

 allied species. 



In some individuals of P. impura, the Sporocysts of C. virgula 

 are large and elongated, and contain a considerable number of 

 Cercarise ; in others, on the contrary, we only find small Sporo- 

 cysts, usually of a rounded form, upon which we can see a sort 

 of more or less apparent umbilicus, and which only contain a 

 very small number of Cercarise (3-4). These Cercarise are ex- 

 actly identical, both in form and organization, with those pro- 

 duced by the large Sporocysts ; but their dimensions are much 

 less, being reduced nearly to half. On examining a great many 

 of these little Sporocysts, we soon see that they are the result 

 of a scission of other larger ones; so that what I have just 

 called an umbilicus really merits that name, because it corre- 

 sponds with the spot at which the separation has taken place. 

 The same difference between large and small Sporocysts exists 

 in the C. microcotyla of Paludina vivipara, as I have pointed 

 out elsewhere t, and this difference now receives its explanation. 

 It is not impossible that these small Sporocysts and Cercarise 

 belong to different species from the analogous large Sporocysts 

 and Cercarise. 



I have been struck this summer by the frequency (although 

 the individuals were always few in number) of a Cercaria, which 

 was first indicated by M. de la Valette under the name of C\ 

 cristataX, and which never occurred to me in my previous re- 

 searches. This singular creature, which I have met with in 

 different species of Mollusca ( Valvata piscinalis, Paludina im- 

 pura, Planorbis submarginatuSy Lymnceus stagnalis, L. palustris), 

 is still the subject of a problem with me. If it be really a Cer- 

 caria, it can only be referred to a Monostomum. 



In the month of August I passed a few days on the shore of 

 the Mediterranean, with the view of making some investigations 



* Second Memoire pour servir a I'histoire eenetique des Trematodes, 

 p. 6. Turin, 1855. 



t Memoire, &c., p. 9. figs. 5, 6 (1854). 



X Symbolae ad Trematodum evolutionis historiara. Berlin, 1854. 



9* 



