in LIVING MATTER 83 



the organisms living in the second, shaded zone, kneplio -plankton, 

 and those living in the third, dark zone, scoto-plankton. 



Since, beside these, there are many organisms which live in- 

 differently on the surface or at the greatest depths, he proposes to 

 collect the four classes together under the collective name of panic- 

 plankton. 



This is not the place in which to enumerate the forms and 

 species of the organisms that constitute, respectively, these four 

 great classes of marine life. We will only state that phao-plankton 

 consists principally of ova that find in this zone the best con- 

 dition for their evolution, and of the larvae or young forms of 

 organisms, which in the adult state live either at the bottom 

 (benthonic forms) or in the deeper strata of the sea. Besides the 

 phao-plankton, certain species of Crustacea (copepods) are abundant 

 The temperature of the water in this zone oscillates from 13 C. in 

 winter to 26 C. in summer. The more or less copious contents of 

 the phao-plankton varies with the seasons ; it is particularly 

 abundant in the spring because reproduction is most active at that 

 season. A striking characteristic of most components of phao- 

 plankton is their minute dimensions. It is a remarkable fact that 

 many pelagic forms are larger in proportion with the depths at 

 which they live. 



The temperature of the second (shaded) zone varies from 13 to 

 24 C. in its superficial stratum, and by a couple of degrees only in the 

 deeper parts, becoming constant (13 C. in the Mediterranean) below 

 that. The plankton which inhabits this region (kneplio -planktoii) 

 is the richest of all. " This zone," writes Lo Bianco, " since it is 

 sheltered from the direct rays of the sun and movements of the 

 waves, is the habitat most favourable to pelagic organisms ; these 

 physical conditions make it the richest and most varied in both 

 plant and animal forms." 



Many varieties of scoto-plankton are brilliant in colour, e.g. 

 most kinds of Crustacea have very red bodies. 



XIII. After discussing the general conditions and external 

 stimuli of cell life, we ought, in another chapter, to consider the 

 general conditions and internal stimuli, i.e. such as arise within 

 the organism itself, and which govern all the vital phenomena that 

 appear to be spontaneous or automatic whether these originate 

 in predominatingly katabolic processes (protoplasmic movements, 

 sensory phenomena, development of heat, electricity, and light), or 

 are due to predominatingly anabolic processes (phenomena of 

 nutrition, reproduction, evolution). Our concrete knowledge of 

 these internal conditions and stimuli is, however, at present so 

 fragmentary that a few paragraphs will contain such general 

 notions as have been determined in relation to them. 



The constitution of a complete elementary organism, capable, 

 i.e., of every kind of essential vital activity, requires that the 



