114 ON THE STRUCTURE OF PROTOPHRYA OVICOLA. 



yolk, of yellowish color, and are enclosed singly in a spherical membrane 200-300 

 micra in diameter. The membrane encloses an albuminous fluid which surrounds 

 the egg (PI. VIII, Fig. 3). The consistency of the membrane and the tension of 

 the contents serve to maintain the spherical contour of the egg throughout the period 

 of development. There is no free veliger stage, and the young snails attain at least 

 two turns of the spire of the shell before leaving the brood-sac. 



It is upon the surface of these egg-membranes that this ciliate is found (Fig. 3), 

 several often occurring upon a single egg; they slide around upon it in ever-changing 

 directions in such a fashion as to remind one of microscopic Canthons with their 

 spherical egg-receptacles. After having been shaken off from the eggs by the 

 process of dissection they will, after standing for a moment in the watch-glass, 

 gather again about the eggs, due possibly to chemotaxis or thigmotaxis, or a com- 

 bination of the two. That the normal and usual habitat of these organisms is 

 upon the surface of these spherical egg-capsules is further borne out by the fact that 

 the arc formed by the curved ventral surface of the ciliate coincides with the curva- 

 ture of the surface of the egg. Repeated examination of the males of the gasteropod 

 host failed to reveal any of these ciliates in or upon the animal save in one instance 

 when a single Protophrya was found in the mantle cavity. The habitat of the para- 

 sites is such that they might readily be transferred to the male at the time of copu- 

 lation, and from the male to other females. 



Associated with the host upon a shingly beach in a cove at the laboratory near 

 the mouth of Narragansett Bay were also Littorina littorea and L. palliata. The 

 latter was more abundant upon brown seaweeds near low-water mark, and the former 

 at higher levels, while L. rudis was present between tide-marks, but more abundantly 

 along high- water levels. Neither of these species are ovoviviparous and Protophrya 

 was not found in or upon them, though a number of individuals of both sexes were 

 examined. In so far as this evidence goes it indicates that this parasite has but the 

 single host. 



The glandular walls of the brood-sac, the suppression of the free-swimming 

 veliger stage of the embryo, and the retention of the young snail until a late stage 

 of development all indicate that a nutritive fluid of some sort bathes the developing 

 eggs in the brood-sac. It is in the fluid of the brood-sac that the parasite lives, and 

 from it the protozoan doubtless derives its nourishment. The absence in the para- 

 site of mouth, pharynx, and anus further supports this opinion, for such food as 

 that suggested would be available by endosmosis. 



The effect of the parasite upon the eggs of the host is seen in the increased per- 

 centage of those which disintegrate in heavily parasitized females. In a few instances 



