70 CONTRIBUTIONS TO AMERICAN HELMINTHOLOGY. 
The structure of the cirrus-coronet is difficult to ascertain on ac- 
count of the semi-opacity of my mounted specimens. The pieces do 
not seem to be more than eight in number; they converge anteriorly 
where they are narrow and pointed; posteriorly they are wider, 
with somewhat arrow-head shaped ends, which fit into the terminal 
bulbous portion of the vas deferens. I have been unable to follow 
the rest of this tube, or to find any trace of the testes. 
Sphyranura resembles P. oblongum and the precocious gill-cavity 
stage of P. integerrimum, in possessing only one complete shell- 
invested egg in the uterus at one time. This is very large (v. 
supra) in relation to the size of the worm, being considerably larger 
than the eggs of either P. integerrimum or P. oblongum. It con- 
sequently forms a noticeable feature in the worms possessing it, and 
is readily detectable with the naked eye. Numerous other eggs 
may be seen in the oviduct formed of the ovarian ova with the in- 
vesting foodyolk-balls, and by mutual compression assuming various 
forms. What I suppose to be the ovary is represented in the figure 
to the right hand of the base of the muscular tube. I cannot find 
any trace of shell-gland, transverse vitello-duct, or of a vagina. All 
of these would undoubtedly be easily seen in fresh or well preserved 
specimens. | 
I regard the genera Gyrodactylus, Dactylogyrus, Sphyranura and 
Polystomum, as forming a very natural assemblage. All probably 
live on the blood of their hosts, beng found in positions where there 
is a more or less close superficial vascular plexus; all possess a caudal 
disc armed with fourteen to sixteen small and two (rarely more) 
large hooks, which enable the fish-parasites to secure themselves 
firmly to the gill-filaments of their hosts. Those which possess 
suckers formed around the smaller hooks are found attached to 
smoother surfaces (mucous membrane of mouth and urinary bladder), 
where the small hooks alone would have little purchase; even these 
forms, however, pass through a suckerless stage in which they inhabit 
the anterior respiratory part of the intestinal tract.” The resem- 
blance of the Polystomum-larva to Gyrodactylus is very striking, so 
that if any phylogenetic speculations may be made from the obser- 
vation of the ontogeny of an animal, the assumption is surely justi- 
30It must be remembered that the mucous membrane covering the hyoid arches of many 
Chelonia has still a high respiratory significance. Vide Agassiz: Contrib. Nat. Hist., U.S. 
Vol. L., Pt. ii, pp. 271-284, 
