502 ON TWO REMARKABLE SPOROCYSTS, 



extremities. In young Sporocysts the internal cavity, owing to 

 the small size of the cyst and the thickness of the germinal 

 epithelium, is extremely small. Occasionally in sections of later 

 as well as earlier stages a delicate reticulated substance appears 

 to till this cavity \vholly or partially : but there can be no doubt 

 that, at least so far as the later stages are concerned, this is an 

 entirely artificial product formed as a result of the coagulation 

 of the fluid contained in the cyst. 



The Mature Cercari^. 



The fully developed Cercaria (fig. 9) reaches a total length of 

 about 4: mm., the tail being included. The head or part of the 

 body bearing the anterior sucker is quite distinctly marked off 

 from the rest, being much narrower. On its ventral aspect is the 

 large cup-shaped anterior sucker, which is capable of being com- 

 pletely everted. Around the anterior margin of the head on the 

 dorsal side, and usuall}'^ inclined outwards and backwards, are a 

 series of flattened tooth-like spines which are capable of being 

 turned forwards. E3'^es are not present. At the sides of the 

 head region are a series of six nearly transverse ridges which do 

 not extend on to the dorsal or ventral surfaces. The remainder 

 of the body is of oval, dorso-ventrally compressed, form; in front 

 of the middle is the posterior sucker. 



The tail consists of a main trunk and two branches. When 

 extended to its utmost the former is over three times the length 

 of the body; the branches are nearly as long as the trunk. The 

 latter expands proximally where it joins the body into a disc, the 

 margin of which is divided by notches and fissures into a series 

 of branched lobes. I have not seen this peculiar structure per- 

 forming any function; but it can hardly be an3^thing else than an 

 organ of adhesion. It contracts and expands slowly at irregular 

 intervals. The main axis of the tail, when at rest, is not in line 

 with the body, but bent towards the ventral side. It performs 

 extremely rapid vibrating movements, propelling the larva swiftly 

 through the water. The tail has a remarkable segmented appear- 



