420 
Marine Cercariae 
parasitic in birds. Others are difficult to place such as Lepodora and 
Cercaria neptuneae, but very little is known of these and further 
investigation will no doubt elucidate matters. When several species 
are known which undoubtedly belong to the same group we find them 
extraordinarily constant as to the kind of hosts they inhabit, thus all 
the members of the EcJiinostomum group so far as is at present known 
develop in rediae in the digestive glands of Gastropods, have a free- 
swimming stage and encyst in another molluscan host, in two cases 
known in the foot. Members of the Spelotrema group develop in 
sporocysts in Mollusca (Gastropods or Pelecypods) and encyst in 
Crustacea, except where the life-history is abbreviated and encystment 
takes place within the sporocyst in the first host. The members of the 
Gymnophallus group all seem to live for a time free between the mantle 
and shell of a Mollusk, usually a Pelecypod. 
It seems to me that we know too little of these cercariae to attempt 
at present to classify them further in relation to their adult forms. I 
therefore have divided up the Prostomata into certain groups each of 
which is characterised in some definite way, but it is not intended to 
show any relationships between these groups other than the general 
resemblance of their larval stages. 
L Gasterostomata. 
Cercaria developed in sporocysts. 
Sporocysts occur in Pelecypoda, long and winding, branched or 
unbranched. 
Cercaria known as Bucejohalus. 
Cystogenous organ atiterior, ventral sucker opening into sac-like 
intestine. Tail a broad case with two lateral, very conti’actile and long- 
filaments for swimming. 
Intermediate host—a fish. 
Final host—a fish. 
II. Prostomata. 
A. (Two suckers.) 
1. Developed in sporocysts. 
(a) Gymnophallus group. Cercaria tail-less, developed (in the 
only species in which the sporocysts are known) in more or less 
spherical sporocysts in Pelecypoda. Oval body covered with spines. 
