THE TREAT A TO DA 53 



Calceostoma, v. Ben. ; C. elegans, v. Ben., on gill of Sciaena agilis. Tetra- 

 onchus, Dies., various species on other fish. Dactylogyrus, Dies., numerous 

 species on fresh-water fish. Amphibdella, Chatin ; Diplectanum, Dies. (Fig. 

 III. 3) ; Gyrodadylus, v. Nordm. ; G. elegans, v. Nordm., on gills of various 

 fresh- water fish (see 22). The most interesting anatomical peculiarity of 

 Gyrodactylus is the absence of a vitellarium, so that, alone amongst the 

 Trematodes, the female gland is an " ovary " ; this is evidently related to 

 the peculiar mode of reproduction, the details of which are still somewhat 



Fio. II. A Group of Heterocotyleans. ^ 



Letters common to all the figures except 3. b, anterior sucker, " lateral " or " buccal," as 

 the case may be ; e, excretory pore ; /, genital pore, or atrium, in 5 ; g, vaginal pore, single or 

 paired ; h, vitellarium, or its duct ; i, intestine ; p, pharynx ; r, germarinm ; 5, sperm duct ; 

 t, testis. 



1. Tristomum coccineum, Cuv. ; ventral view (altered from Bronn's figure). , mouth, lead- 

 ing into the pharynx ; c, the large posterior sucker, whose cavity is subdivided into a central 

 and seven peripheral "loculi," by a circular and seven radial muscular ridges ; a )>air of small 

 hooklets (scarcely visible in the figure) are carried by the pair of posterior radii. The intestine 

 consists of a subcircular main canal, bearing many much-branched caeca. The canal is only 

 represented in outline, and the caeca only partially indicated on the right of the figure. The 

 horseshoe-shaped vitcllarian duct and the branches therefrom follow the course of the corre- 

 sponding parts of the enteric canal. From the anterior end of each limb of the main duct a 

 transverse duct (represented only on the right of the figure) passes inwards to join the rest of 

 the female apparatus, which is not represented. 



2.Hexniv>tule grossa, Goto, from the gill of Tliynnus sp. (ventral view, after Goto). There is 

 a long oesophageal region (o) passing back from the pharynx to the level of the genital pore, 

 behind which it bifurcates. Each limb gives rise to a series of anastomosing branches forming 

 a marginal network shown on the left of the figure, which is continued posteriorly into the 

 caudal disc ; and behind the testes a central network is similarly formed. The pore of the 

 vagina lies on the rtorsnl surface ; the vagina itself bifurcates posteriorly, indicating its true 

 double origin. 1, 2, 3, the three large suckers, and 4, the minute median sucker, represented 

 by a dot on the caudal disc. Between the latter are two small hooklets, represented by short 

 lines. 



3. The female organs of Sphyranura nsleri, Wright, from Necturtts lateralis, ventral view. 

 The genital atrium (/) receives 'the male duct () anteriorly and the female duct (^pos- 

 teriorly; d, uterus, dilated distally and containing an egg (o) which is provided with a 

 filament at one end ; /, vagina, right and left, here a blind sac, having lost its external pore and 

 functioning as a spermatheca ; h, right and left, transverse vitelline duct ; h', yolk reservoir 

 (? = ootype); i, portion of the intestine ; fc, genito-intestinal canal ; q, germ duct ; r, gcrmarium; 

 a;, the point of union of the various ducts. 



4.Didulophora elongate, Goto, showing the alimentary system. The two limbs of the 

 intestine are united by several transverse branches, both in front of and behind the genital 

 organs (which are not represented). Caeca are also given off laterally, and one enters the stalk 

 of each posterior sucker. 1, 2, 3, 4, the four suckers at the left side, each is armed and pedun- 

 culated. 



5.Udonclla cdlignrum, Jnstn., from CoHpis (after P. J. van Beneden). ft, the anterior 

 sucker surrounds, as by a collar, the everted pharynx. The posterior sucker is a simple deep 

 cup. The uterus contains an egg anteriorly. The right ami left lobes of the vitellarium are 

 united posteriorly by a median lobe. The transverse vitellarian ducts are seen ]>a.ssing to 

 the ootype, which receives also the germ duct from the germarium. The intestine, here, is a 

 simple bifurcated tube, without caeca. 



G.Anthocotyle merlncii, v. Ben. and Hesse, from the gill of Jferfaefmi imJgarb (after 

 Cerfontaine). Outline showing the ventral surface, pores, and suckers. 1, the large clasping 

 sucker of the posterior system, one valve alone, with its armature, is shown : 2, 3, 4, the three 

 small suckers ; w, the terminal region of the disc (enlarged on the right), with its two pairs of 

 hooklets. 



obscure. A single egg ripens at a time, and develops into an embryo while 

 in the uterus ; but within this embryo a second embryo becomes marked 

 out before the first leaves the mother (Fig. III. 6). There are thus three 

 generations, one within the other ; and Wagener (46) has suggested three 

 possibilities to explain what happens, but is unable to decide between them. 

 (ft) The grand-daughter arises, like the daughter, by the ordinary sexual 

 process (which is very improbable), (b) Some of the blastomeres of the 

 original egg remain quiescent and take no share in the formation of the 

 daughter, but later undergo development within it to form the second 



