﻿250 Proceedings of the Royal Physical Society. 



Psammoryctes costatus, however, occurs as far down 

 as Zone 3. 



PoLYCH/ETA. — They seem to be very sensitive to fresh water, 

 for only one of them, Spirorhis horealis, occurs in a 

 position exposed to fresh water. Perhaps the operculum 

 enables Spirorhis to resist the deleterious effect of fresh 

 water. Most of them occur in Zones 3 and 4. Poly- 

 chsets are among the most characteristic inhabitants 

 of Zone 4. 



PoLYZOA. — While the majority of the forms occur in the 

 lower reaches and avoid contact with fresh water, 

 Alcyonidium mytili, as has been mentioned above, 

 occurs both high up and in fresh water. Fucus serratus 

 seems to be a favourite position for Polyzoa. Bower- 

 hankia imbricata we have found only upon Oladophora. 

 The favourite position occupied by Memhranipora mem- 

 hranacea is of course the laminae and stem of Zaminaria, 

 but it is found also on F. serratus. 



Pelecypoda. — Modiolaria discors is found typically in a 

 little nest made of byssus threads on the under surface 

 of stones. 



Mytilus editlis, we have noticed, is most abundant 

 in Zone 2, Upper. Minute specimens were frequently 

 found sheltering in empty barnacle shells. 

 Gastropoda. — The familiar ecological relationships of 

 Acmaea testudinalis, A, virginea, Patella vulgata, and 

 Helcion pellucidvm are clearly brought out by the 

 Tables. 



Gihhiola cineraria and G. umhilicata, Littorina rudis 

 and L. ohtusata : — The occurrence of closely allied species 

 side by side is exemplified by these forms. 



L. rudis shows a certain response to the humidity of 

 the air by coming out upon the surface of stones in Zone 

 1 in greater numbers on wet days than on dry days. 



Doto coronata was found as usual on a tuft of Coryne, 

 which it resembles greatly. Is this association really 

 protective ? Experiment alone can answer this question. 

 Amphipoda. — In this group we meet with a striking case of 

 the constant association of one species with another, 



