334 SECTIONAL TRANSACTIONS.— D. 



new sources of food. There are well-marked correlations between 

 (i) habitat and available food of any animal ; (2) type of feeding mechanism ; 

 (3) structure of the gut; and (4) nature and relative strengths of the various 

 digestive enzymes. In the Protozoa feeding mechanisms of all types are 

 present but in all cases digestion is intracellular. Metazoa probably evolved 

 from Protozoa with least specialised feeding mechanisms. Porifera are a 

 special case. The evolution of extracellular digestion, at first assisting 

 intracellular digestion — e.g. Coelenterata, Turbellaria, Echinodermata, 

 many Mollusca — and finally completely replacing this, has enabled animals 

 (i) to reduce size of alimentary system ; (2) to complete digestion more 

 quickly ; and (3) to remove indigestible matter more quickly. Metabolism 

 is greatly assisted and the animal more efficient. All the most active groups 

 of animals, e.g. Annelids, Arthropods, Cephalopods, Vertebrates, digest 

 extracellularly. These animals have most successfully colonised the land. 

 If feeding mechanisms and digestive system too specialised, e.g. Brachiopods 

 and Lamellibranchs, further evolution almost impossible. Loss of digestive 

 system in parasitism leads to degeneration. Development of special 

 enzymes permits of the utilisation of new sources of food, e.g. cellulase in 

 Teredo and wood-boring Insecta, or greater success in particular habitat, 

 e.g. cellulase in Strombidae and Helix, chitinase in some Insecta and in 

 Helix, enzyme for digesting collagen in blow-fly larvae. 



Prof. F. Balfour- Browne. — The colouration of the cocoons of the Small 

 Eggar moth {Eriogaster lanestris, L.). 



In 1886, Poulton described experiments with the caterpillars of the 

 ' Emperor Moth,' Saturnia carpini, which, when placed in a black bag, 

 formed dark brown cocoons, while caterpillars freely exposed to the light 

 with a white environment produced white cocoons. These results he de- 

 scribed as due to colour susceptibility on the part of the caterpillars. 



In 1892, Bateson, having repeated these experiments and failed to confirm 

 them, showed that pale-coloured cocoons were as frequent in the dark as 

 in a well-lighted environment. Some pale cocoons, such as those described 

 by Bateson as ' thin and papery,' are abnormal, but most white and cream- 

 coloured ones are as thick and firm as the dark ones, and the colour of the 

 cocoon is due to the dryness or dampness of the environment in which the 

 caterpillar spins up. Pale cocoons are formed under dry conditions, and if 

 placed in a saturated atmosphere, even after the caterpillar has pupated, 

 these quickly darken to the normal brown colour. 



Therefore Poulton's suggestion that the colour is protective is reasonable, 

 since the caterpillars spin up amongst dead leaves, etc., often below a hedge- 

 row, and the dampest places are likely to be those most sheltered and there- 

 fore the darkest, whereas the drier places, where the dead leaves are of a 

 lighter tint, are more exposed to light and air. 



Dr. J. Gray, F.R.S. — The mechanism of animal movements, with special 

 reference to fish. 



Three main types of propeller are found in the animal kingdom : (i) jet 

 propellers, e.g. the siphons of squids ; (ii) paddles, e.g. the webbed feet of 

 swimming birds ; (iii) inclined blades. Of these types by far the commonest 

 is the inclined blade wherein the propulsive elements move as surfaces 

 inclined to the direction of their own motion through the water. In this way 

 pressure is exerted on the blade at right angles to its surface ; this pressure 



