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REPORT ON ZOOLOGY, MDCCCXLII : 
witli the tailless Cercaria echinata, the resemblance of these animals, 
especially of the head and wreath of hooks, is very striking ; and if we 
remember the interesting observation of Creplin, that the Schistoce- 
phalus dimorphus (Nov. Observat. de Entoz. p. 90) first gains its 
sexual organs, after it has been transplanted from the stickleback into 
the intestinal canal of the water fowl, we must be led to assume, that 
the sexual organs of the pupa of Cercaria echinata do not become com- 
pletely developed until it has arrived in the intestines of those water- 
fowl which feed on snails, and then grows up to one of the above named 
armed Distoma. Steenstrup next turns to the metamorphoses of the 
Cercaria armata, Sieb,, on the anterior part of the body of which he saw, 
on both sides, a serpentine organ, which, as the reporter conjectures, 
discharges itself at the anterior end of the body, as in Cere, echinata ; 
perhaps the same opening serves for an exit to both those secreting 
pouches, from which the point of the prickle of the head projects. The 
forked intestinal canal of these Cercarice, and the posterior excretory 
organ, have not been mentioned by Steenstrup. He saw the shells of 
Lymneeus stagnalis and Planorhis corneus, not merely surrounded by 
monstrous swarms of these Cercarice, but also their bodies thickly 
covered by them. The Cercarice creep about upon them, and penetrate, 
by means of their prickle, into the cuticle, that they may cast off their 
tail ; when this takes places, the inner caudal tube is constricted, and 
receives, according to Steenstrup’s account, an aperture outwards, through 
which the animal then presses out a fluid, filled with globular masses. 
The reporter does not take this view. The cavity which contains the 
granular fluid does not belong to the tail of the Cercarice, but is the 
very short forked excrementary organ, situated in the posterior part of 
the animal’s body, the aperture of which is stopped up by the root of the 
tail, as in the other species of Cercarice. After they have penetrated 
into the cuticle of the snails, they become pupae, while they excrete a 
slime from their whole surface, and at once strip off their upper skin 
together with the prickle. The Cercarice remain very long active and 
fresh in this state of pupa. We cannot help wondering how Steenstrup 
could consider the tailless Cercaria armata changed into a Distomum 
quite reversed. He describes its short forked posterior excretory 
organ (tab. iii. fig. 4 f. u. g., 4 d. u. e., a. y.) as the intestinal canal ; and 
the double caecum, going right and left from the head of pharynx (tab. iii. 
fig. 4 f. u. g., V. X. X.), he considers as an organ for the service of propa- 
gation. The great acetabulum of the mouth (tab. iii. fig. 4 f. u. g., 4 d. u. 
e. s, t.) is to him quite a mystery ; as, misled by optical delusion, he looks 
upon the section of the muscular walls as a peculiar horse-shoe-formed 
organ, and the proper opening of the mouth as the aperture of the pos- 
terior excretory organ. A Distomum, which had slipped out of its 
covering (tab. iii. fig. 5 a.), he also got hold of in the same way reversed ; 
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