Summary. (1) The threshold for discharge in response to mechanical stimulation of the stenoteles and desmonemes of Hydra is lowered by food extracts, in the same way as has been shown by Pantin to occur in sea anemones. (2) The tentacles are attached during locomotion by discharge of the atrichous isorhizas. Food extracts have the effect of inhibiting the discharge of this type of nematocyst. The duration of mechanical stimulation necessary to bring about discharge is greater for atrichous isorhizas than for stenoteles. (3) The holotrichous isorhizas have only been found to be discharged when stimulated by animals which are not normally eaten by Hydra and evoke no feeding reaction. Their threshold is unaffected by food extracts. Their function is concluded to be defensive. (4) Normal feeding and locomotion in Hydra are described, and the relation of the reactions of the nematocysts to chemical and mechanical stimuli in their normal functions is discussed. My thanks are due to Professor H. Munro Fox, F.R.S. in whose laboratory this work was carried out, to Dr. C. F. A. Pantin, F.R.S., for helpful discussions and advice, and to Messrs. George Newnes Ltd., for permission to reproduce fig. 1.

Summary. This is the first comparative study of the activity and metabolism of poikilothermal animals living in different latitudes. For seven pairs of species of marine invertebrates, inhabiting respectively English waters and more northern seas, the oxygen consumption of the warmerwater species is greater than that of the colder‐water species (measured at the normal temperatures at which each species lives) although the locomotory activity of the former is apparently no greater (Table I., text‐figs. 1 & 2). It is suggested that the similar locomotory activities of the two species require approximately equal amounts of oxygen, but that the non‐locomotory oxygen consumption of the warm‐water species is higher than that of the cold‐water species. The rate of ciliary movement on the gill of the scallop Chlamys varia from English waters is higher than that of C. septemradiata from more northern waters, each measured at the temperatures at which the animals live (Table II., text‐fig. 3). The respiratory movements of English species of Crustacea are no more rapid than those of corresponding species in more northern waters (Table III., text‐fig. 4). The rate of heart‐beat has also been compared in species of Crustacea in English and more northern waters (Table III., text‐fig. 5). The rate of scaphognathite‐beat of the prawn Pandalus montagui is no more rapid in English than in more northern colder waters (Table III., text‐fig. 4). This is a case of variation within a single species in seas of different temperatures; it is parallel to what has been described above for pairs of different species inhabiting colder and warmer seas respectively.

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