390 T. B. ROSSETER ON H0L0ST0MUM EXCISUM, AND THE DEVELOPMENT 



Whilst agreeing -with Brandes that the most positive evidence 

 of the evolution of a Holostomid from a tetracotyliform larva 

 is only to be arrived at by successive experimental feeding of a 

 known host with the larvae, and thus raising the sexually mature 

 animal, still, in the absence of such an experiment having been 

 accomplished in the past, and the want, so far as I am concerned, 

 of material to enable me to undertake a series of experiments 

 with such an ulterior object, having obtained what, in my opinion, 

 are obviously the several stages of its development from a Tetra- 

 cotyla to the mature worm, I feel justified in publishing these 

 facts as a continuation of von Linstow's work. 



In this species of Holostomum the ovary (Fig. 10, o.) lies in 

 close proximity to the neck. It is partially covered posteriorly 

 by the receptaculum seminis (Fig. 10, r.s.). Both of these are 

 semi-globular. The latter is the swollen sac of the sinuous laurer 

 canal, whose pore performs the function of a vagina for the 

 reception of the sperm (Figs. 10 and 16, l.c). The shell gland 

 (Fig. 10, s.G.) is oval, and occupies the median vertical portion of 

 the body. The vitellaria (Figs. 5-13) extends from the head to 

 within a short distance of the caudal end. It is located dextrally 

 and sinistrally in the cavity of the body, and partially covers — 

 and by the density of its pigmentary glands conceals — the whole 

 of the genital tract. The yelk duct runs vertically over the shell 

 gland. Its centre is swollen, and forms a sac, the yelk reservoir 

 (Fig. 10, y.r.), whilst its terminative ends, proximally and 

 distally, taper off into thin filiform ducts, which curve on either 

 side and then bifurcate. Into these are poured the yelk substance 

 from the glands, it being then passed on to the reservoir. The 

 ovarian duct passes laterally posteriorly, dorsal of the anterior 

 testis, and previous to its entering the shell glands makes a 

 junction with the ductus efferentia (or fertilising canal). The 

 uterine canal, when it leaves the shell gland, descends to the 

 dorsal cavity, along which the uterus is formed and into which 

 the golden-coloured eggs are passed (Fig. 10, u. and ova). There is 

 never at any time a multiplicity of eggs deposited in the uterus. 

 At the proximal pole of the egg there is an operculum. Each egg 

 has a polar length of 0"084 mm., and transversely measures 

 0*05 mm. 



There are two ellipsoidal testes. They are separated anteri- 

 orly and posteriorly by the shell gland (Fig. 10, T.) I have not 



