456 SUMMARY OF CURRENT RESEARCHES RELATING TO 



7. Gastropoda. 



Migration of Retinal Pigment in Planorbis.* — L. B. Arcy finds 

 that light causes the pigment to migrate distally (towards light) in about 

 five hours ; darkness causes proximal migration in five hours. Light 

 produces the same effect on excised eyes as on eyes in situ. Warmth 

 (30° C.) has the same effect as light ; cold (3° C.) the same effect as 

 darkness. 



Structure of an Eolid.f — Harold Heath describes CMoraeradalUs^. n., 

 a littoral Eolid from British Columbia. The various systems of organs 

 are surrounded by a loose meshwork of connective-tissue and muscle- 

 fibres, with the intercommunicating spaces serving as blood sinuses. An 

 account is given of the alimentary system (without trace of jaws, radula, 

 or distinct salivary glands ; with a digestive gland generally distributed 

 throughout the body) ; the vascular system, which is mainly lacunar, the 

 aorta disappearing soon after it leaves the heart ; the kidney, with a 

 long thin-walled reservoir ; the reproductive system ; and the nervous 

 system. 



Varieties of Dog-whelk.| — Harold S. Colton has made an ecological 

 study of Thais {Purpura) lapillus in the neighbourhood of Mount Desert 

 Island, Maine. Over 12,000 whelks were collected from sixty-seven 

 localities, sorted, and the variations tabulated. These tables were then 

 compared with the environment of the whelks. 



The whelks are limited in number by the following factors : — 1. 

 Cannibalism within the egg-capsule, only ten to twelve hatching out of 

 300 to 400 eggs. 2. The available cracks in the rocks with sufficient 

 food. 3. The predatory attacks of fishes, such as pollacks. 4. The use 

 of half-grown whelks by herring-gulls, which regurgitate the shells. 

 5. The destruction of large specimens by gulls, which drop them on 

 flat rocks (as is also done with Buccinum and Strongylocentrotus). 6. The 

 attacks of a parasitic sporocyst. 7. Accidents due to movements of rocks 

 by the surf. 8. Destruction by ice. 



Among the adaptations may be noted the strong shell, the strong 

 adhesive foot, the habit of sheltering under rocks and Fucus, and the 

 method of egg-laying. There are many colour varieties — white (with 

 intense and dilute chestnut), chestnut, purple, pure yellow, striped, etc. 



The colour varieties seem to be hereditary, and probably unit cha- 

 racters. The formation of lamellae, as in var. imbricata, is not due to 

 environmental action. Though the surf environment is the antithesis of 

 the harbour environment, the effects on the whelks are in some respects 

 similar, thus the growth is slow^ei' than in the optimum bay environment, 

 where no mud gathers ; there is a tendency to dark colour and to 

 lamellation. 



Natural selection has some determining action on the colour varieties 

 present. The colour of the rocks, whether light or dark, has an effect. 



* Journ. Comp. Neurology, xxvi. (1916), pp. 359-90. See Physiol. Abstracts, 

 i. (1916) No. 8, p. 349. 



t Proc. Acad. Nat. Sci. Philadelphia, Ixix. (1917) pp. 137-48 (3 pis.). 

 t Proc. Acad. Nat. Sci. Philadelphia, Ixviii. (1917) pp. 440-54 (4 figs.). 



