88 THE TREMATODA 



covered by Laurer (1830), and since known as "Laurer's canal" 

 (Stieda. 1867), which passes up from the oviduct, in the neigh- 

 bourhood of the ootype, to the dorsal surface, upon which it opens 

 by a minute pore (Fig. XXII. 1, /). It has been till recently 

 regarded as the homologue of the vagina of Heterocotylea ; 

 but it does not function as such ; it is much too small for the 

 insertion of the penis, and although spermatozoa have been 

 observed in it (Looss, 31), they are passing outwards, in which 

 they are aided by the current produced by the cilia lining the 

 canal; yolk granules also occur. Recently both Looss and 

 Goto independently have brought forward various arguments from 

 comparative anatomy tending to show that Laurer's canal of the 

 Malacocotylea is the homologue of the " genito-intestinal canal " 

 of the Heterocotylea, which has lost its connection with the 

 intestine and come to open to the exterior ; for it would be 

 manifestly less important for an endoparasite to make use of its 

 yolk as food than for an ectoparasite. 



(4) In Aspidocotylea the duct leading to the " receptaculum 

 vitelli" agrees closely with Laurer's canal and the genito-intestinal 

 canal, except that it ends blindly below the dorsal integument 

 (Fig. XXII. 2) ; and there is little doubt that these three canals 

 are homologous. 



The eggs of the Trematoda which are operculate, are of various 

 shapes and have a certain systematic value ; in the Malacocotylea 

 they are very numerous, much smaller than in Heterocotylea, and 

 rarely have the filament which is so usually present in the latter 

 order, since in the endoparasitic forms they are generally dis- 

 charged to the exterior and not attached to the host, as in ecto- 

 parasitic forms. In a few instances, however, one filament exists, 

 as in Bilharzia and species of Distomum, or less frequently two in 

 D. constrictum, Monostomum verrucosum, Opisthotrema cochleare. 



Historical. The earlier writers, who concerned themselves chiefly 

 with the liver-fluke, frequently confused the excretory canals with the 

 intestine ; thus Carlisle, having injected the former, described it as the 

 latter, the excretory pore being termed "porus ani." Even after the 

 true mouth and intestine had been correctly recognised by Ramdohr 

 (1814), and by Bojanus (1817), there was a general belief that the excre- 

 tory pore served as an anus, and that the excretory canals were in connec- 

 tion with the intestinal caeca, and acted as a kind of vascular system ; 

 indeed, Blanchard (1847) described the contractile bladder as a "heart," 

 and went so far as to deny the existence of a posterior pore. Bojanus 

 (1821) was the first to establish the absence of an anus, though even v. 

 Baer (1827) mistook the excretory pore of Aspidoyaster for the mouth. 



The excretory system, even after it had been differentiated from the 

 enteric system by Bojanus, and by v. Siebold, who pointed out its true func- 

 tion, was variously regarded as (a) respiratory by Meckel (1846), who sug- 



