THE TREMATODA 57 



Arrived in the gill chamber, the larva undergoes a gradual meta- 

 morphosis, the cilia disappear, and the cells bearing them shrink 

 up ; suckers make their appearance in pairs on the caudal disc, 

 each being formed around a booklet, thus leaving ten booklets 

 outside the suckers. Meanwhile, the tadpole itself is undergoing 

 metamorphosis, and the young Polystomum makes its way into the 

 pharynx, and wanders along the alimentary tract to the rectum ; 

 on the formation of the cloacal bladder, the young Polystomum 

 enters it. It is not till the third year that the parasite 

 becomes sexually mature. But if, as sometimes happens, 

 the larva of the parasite has attacked a younger tadpole than 

 usual, one in which the external gills are still present, it can 



FIG. IV. Diplozoon paradoxum, v. Nordm., from the Minnow. (Figs. 2-S after Zeller.) 



1. The complete animal pair (altered from v. Nordmann) seen from the ventral surface. 

 The individual A A 1 is permanently copulating with Bit'. The intestine is shown in each ; it 

 consists of a single tube produced into caeca ; really on all sides, but appearing here only on 

 right and left. Beyond the point of union of A A ' with BB 1 , and in the region occupied by the 

 genital system, their lateral caeca are absent, but they reappear again behind the testes. In 

 D. nipponicum the intestine bifurcates at the cross and surrounds the genital organs, behind 

 which the two limbs again unite. <j, caudal disc, with four suckers upon it. 



2. A pair enlarged to show the genital organs. AA ' is seen from the ventral surface ; 

 BB' is truncated, and seen from the dorsal surface, o, the mouth; b, buccal sucker; c, 

 pharynx ; d, intestine ; e, vitellarium of AA 1 ; /, vitellin* duct ; /', its dilated region in the tail ; 

 it enters the germ duct just beyond the origin of the "vagina"; g, testis of BB'; h, sperm 

 duct, which can be traced forwards into the anterior part of the animal, curves round and com- 

 municates witli the vagina (i) of A A 1 , which is seen passing ventrally below the germarium. 

 k, the germarium, the hinder end of which is produced into the transversely disposed germ 

 duct which receives the vagina and vitelline duct, and is continued forwards to form the uterus 

 (I). The enlarged part (I 1 ) of the uterus is lined by large cells ; the anterior part, which runs 

 transversely, is termed by Zeller "egg duct" ; it opens to the exterior at (m) the female pore 

 situated on a slight papillae and communicating Avith I by a narrow duct n. p, the caudal disc, 

 dorsal surface in B 1 , bearing below it (in A Q the four suckers (</), each of which is armed. 



3. An egg shell, a, operculum ; b, filament. 



4. Young ciliated larva, b, buccal sucker ; c, pharynx ; d, intestine ; Q, the most anterior 

 sucker, the only one yet developed. 



5. Older larva, after the loss of cilia, and now known as Diporpa. On its ventral surface a 

 special sucker (e) has developed. 



6. Two Diporpae about to unite. The ventral sucker of A is seizing the dorsal papilla 



-- 



7. The two Diporpae are twisting round, so that the ventral surfaces of the "tails" are 

 towards each other. 



8. The two Diporpae have arrived at the definitive position, forming a cross, and constitut- 

 ing a Diplozoon. 



obtain so copious a supply of nutriment that it grows extremely 

 rapidly, and produces eggs in five weeks; it remains, however, in 

 the gill chamber when this is formed, and dies before the meta- 

 morphosis of its host. Thus " gill parasites " differ in several 

 points from the " cloacal form," notably in the absence of a vagina, 

 and consequently in the impossibility of copulation. 



ORDER 2. Aspidocotylea, Monticelli ( = Aspidobothrii, Barm.). 



Trematoda in which the adhesive apparatus has the form of a great 

 sucker occupying nearly the entire ventral surface of the body, from 

 which it is usually distinctly constricted off. This single sucker becomes, 

 during development, traversed by transverse (and usually also longitudinal) 

 ridges, and is thus in the adult subdivided into a number of more or less 



