300 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the slime secreted ; (b) the relative proportions of the length of the 

 anterior and the posterior regions of the animal's body. All the condi- 

 tions being the same, it is this last factor which determines whether the 

 head end will be directed up or down. 



If the ratio of length of the anterior region (from the tip of the 

 head to the posterior edge of the mantle) to the posterior region is 2 : 3, 

 or more, and if the mucus is of good quality and sufficient quantity, the 

 slug will be positively geotactic. If the ratio is 3:5, or less, the 

 animal will usually migrate upward, and the nearer the ratio approaches 

 1 : 2 the more apt is the slug to respond negatively. In a small number 

 of individuals, in which the ratio lies between 2 : 3 and 3 : 5, the re- 

 sponse will depend largely on the condition of the mucus and co-opera- 

 tion of other factors. 



All sln^s have a natural tendency to move towards the earth. This 

 tendency is masked in the animals which are negatively geotactic on a 

 glass plate by the greater pull of gravity on the disproportionately 

 larger and heavier posterior region of the animal. The general down- 

 ward tendency may vary normally at different times of the day, owing 

 to the animals' habit of remaining in concealment in the daytime and 

 feeding at night. 



(III.) The slugs are markedly phototactic, but with individual dif- 

 ferences, as in geotaxis. To strong light, they give, on the average, 

 a strong negative response, and the degree of response gradually 

 diminishes with the reduction in the strength of the stimulus. Thero 

 is a certain strength of light — neutral stimulus — which appears neither 

 to repel nor to attract, and reduction of the intensity beyond this neutral 

 point changes the phototaxis from negative to positive. The positive 

 response becomes stronger up to a certain degree of intensity ; it then 

 gradually diminishes with decreasing intensity until absolute darkness,, 

 accompanied by no response, is reached. Slugs are responsive to light 

 stimuli covering a wide range of intensities. The principal organ of 

 response is probably the eye. The right side is not as sensitive to 

 stimuli as the left. In the dark, other directive stimuli being eliminated, 

 the slug tends to travel in a spiral of gradually increasing radius, 

 though almost invariably producing one or more loops. There is a 

 slightly greater tendency to right-hand circling. 



" These responses of the slug to touch, gravity, and light-stimuli 

 emphasise the fact that it is an animal's normal environmental condi- 

 tions which chiefly determine its general response to artificial stimuli. 

 The variations in precision and character of this general response are 

 mainly dependent on certain internal factors, such as the food conditions 

 of the animal, its fear of an enemy, and desire to escape captivity." 



Digestive Gland in Molluscs.* — P. Enriques has made a detailed, 

 chiefly microscopical, study of the digestive gland in Aplysia depilan* 

 and limacina, in comparison with Pleurobranchsea meckelii, the common 

 snail, the oyster, and various Cephalopods. His results include some 

 details in regard to the structure and relations of the digestive gland, an 

 account of the mechanism of trituration and digestion in Aplysia, and a 

 description of the microscopical changes exhibited by the food-particles^ 



* MT. Zool. Stat. Neapel, xv. (1901) pp. 281-107 (3 pis.). 



