ENTOZOA — TREMATODA. 
311 
he now assumes of the Cercarice become pupae and changed into Distoma, 
that they grow strong after becoming pupae, and acquire a lanceolate 
shape, as the fore-part of the body is strongly contracted, and the pupal 
covering at the same time much thickened. The very thick skinned 
Helminthes, to which Steenstrup next refers, and which he has figured 
on tab. iii. fig. ‘5 e. f. u. g., do not belong to the series of the metamor- 
phoses of Cercaria armata. These Helminthes are very remarkable 
Trematoda, without sex, which the reporter has often met with between 
the nests of Cercarice in Planorbis and Lymnceus ; the irregular net of 
canals, which contains a granular fluid, is the very much dilated excre- 
mentary organ of these Helminthes, and widely difierent from the simple 
short forked one of the Distoma which come from Cercaria armata. 
Between the acetabula of the mouth and abdomen of these animals two 
semicircles are observed, which Steenstrup considers as the broad ends 
of the cavity of digestion ; but they are two openings obliquely perfora- 
ting the thick cuticle, and leading to a groove in the parenchyma of the 
worm. Steenstrup has been more happy in pursuing the metamorphoses 
of Cercaria retrogressively ; he has succeeded in recognising, in their 
matrices, which have hitherto been esteemed as immoveable simple 
pouches, a slight voluntary motion, as well as a sort of acetabulum at 
the one end, and an opening for parturition at the other. 
In their youngest state, they contain a cellular mass ; in the gradual 
development of the Cercarice the motion by degrees ceases. Steenstrup, 
as well as the reporter, has observed the Cercarice becoming pupae be- 
fore they leave the matrices, and appeals to the large thick-skinned Tre- 
matoda which he has found in the matrice-bags. These Helminthes have 
also been seen by the reporter in the matrice-bags of the Cere, armata, 
as well as echinata, and must, as already mentioned, be considered as 
parasites not belonging to the series of metamorphoses of these Cercarice. 
Steenstrup has not hitherto been able to perceive the first matrices 
(Gross-ammen) of the Cercar. armata, that is, such matrice-bags as 
contain young pouches ; he farther mentions, that he obtained several indi- 
viduals of one Distomum from the liver of a Paludina vivipara, which 
he holds for the same species into which the Cercar. ephemera, Nitzsch, 
changes on becoming pupa. This is not likely, as the Cercar. ephemera 
wants the abdominal acetabulum ; and, accordingly, this larva could only 
be changed into a Monostomum. Nitzsch has erroneously ascribed an 
abdominal acetabulum to this Cercaria. Steenstrup next describes a 
small oval animal, which moves by vibratile cilia, and in all respects 
resembles the brood which comes from the eggs of Distoma, and first 
becomes transmuted into a Distoma-Vike animal in the third generation. 
It lives in the internal organs and external slime of Anodonta, and much 
resembles a Paramecium. The individuals gradually lose their vibratile 
cilia, fix themselves, and become more parenchymatous ; as they grow, 
a cavity is formed in their interior, which becomes, by degrees, filled 
35.5 
