ENTOZOA — TREMATODA. 
307 
Steenstrup first made his observations on Cercaria echinata, Sieb., 
from Planorhis cornea, and Lymnceus stagnalis, and described it so 
exactly, that the reporter knew it at once to be the Cercaria first men- 
tioned in Burdach’s Physiology. The internal cavity, with contractile 
walls in the after part of the body, he incorrectly looks upon as the end 
of the root of the tail. This is situated not nearly so deep in the after 
part of the body of the Cercarice, and only encloses the opening of the 
cavity mentioned. According to the reporter’s observation, the wide- 
extended aperture, at the root of the tail, contracts itself when that 
part falls off, and then represents the spot for the discharge from the 
excrementary organ. The smaller circular speck, which is situated 
before the cavity, and looked upon by Steenstrup as an opening, is only 
a spot enclosed by the contractile walls. Steenstrup has made the fol- 
lowing remarks on this Cercaria echinata : — In swimming, each Cer- 
caria rolls its body into a ball, approximating the head to the tail-end, 
and beats about with the elongated tail in innumerable S-shaped figures. 
These larvae swarm for some time around the snails from which they 
have come forth, and fasten themselves, by means of the abdominal 
acetabulum, to their slimy cuticle, and then stretch out the fore and 
after part of the body. After a time they begin, with leech-like mo- 
tions, to creep about upon the cuticle of the snails ; when again they 
become fixed, and by wriggling strongly up and down, they endeavour 
to loosen the tail. When they have succeeded in this, it dies away, and 
the Cercaria assumes the appearance of a Distomum. During the 
attempts to cast otf the tail, an abundant secretion of slime exudes 
from the whole upper surface of the body ; and the tail being cast otf, 
the worm, by manifold movements and turnings, makes for itself a cir- 
cular cavity in this slime, that gradually thickens into a case around it. 
Steenstrup is of opinion, that in doing this, the animal denudes itself of 
a very thin cuticle, which the reporter is inclined to doubt. The former 
grounds his opinion on this, that after the formation of this covering, all 
the internal organs of the Cercaria become more distinct ; but the 
reporter would explain this fact, by the emptying of the slime-glands 
of the cuticle making the body of the animal more transparent. This 
Distoma-\SkQ worm possesses, at the anterior end of its body, a sort of 
cape, which is deeply emarginate on the middle of the abdominal side : 
on this stand the simple prickles or needles, from which the Cercaria 
derives its specific name. They are situated at greater and lesser dis- 
tances, alternating regularly, in a double concentric circle, the pointed 
ends directed outwards, and the blunt ends inwards to the mouth. The 
large abdominal acetabulum is placed somewhat behind the middle of the 
body. Steenstrup next describes a large bladder-like organ in the interior 
of the Cercarice, which begins close to the margin of the cape, runs down 
to the abdominal acetabulum in the middle of the neck, and then splits 
351 
