10 THE PHYSIOLOGY OF REPRODUCTION 



year, in members of the same class in northern seas it only 

 lasts through the summer. 1 



NEMERTEA, &c. 



The breeding season and its relation to the environment 

 have formed the subject of a careful investigation by Child 2 

 in the case of a small Nemertean, Stychostemma asensoriatum, 

 which is found very abundantly in one of the park lagoons of 

 Chicago. The season extends from May to November or 

 December, according to the temperature of the water. Egg- 

 laying can occur freely in the laboratory, the eggs being de- 

 posited always during the night, or in darkness, when the animals 

 move about freely. Although breeding in the natural state is 

 restricted to the warmer part of the year, eggs can be obtained 

 in the laboratory at practically any time, by simply regulating 

 the temperature. Thus egg-laying can be induced in the winter 

 at ordinary room-temperature, even though the worms are kept 

 without food. " In animals which contained only a few small 

 oocytes when taken, and which are kept in clean water without 

 food, the growth of the oocytes will continue, and within a 

 week or two eggs may be laid/' " The body of the animal 

 may even decrease somewhat in size during the growth of the 

 oocytes." It is clear, therefore, that in Stychostemma the limits 

 of the breeding season are determined chiefly by the temperature 

 of the water, and that food is a factor of secondary importance. 



Similarly, in. the case of the parasitic Trematode, Diplozoon 

 paradoxum, which ordinarily produces eggs only in the summer, 

 it has been found that the formation of eggs could be artificially 

 prolonged throughout the winter, if the fishes on whose gills 

 the animal lives are kept in an aquarium at summer heat. 3 



ANNELIDA 



Certain species of Polychset Annelids, known as the Palolo 

 worms, exhibit a quite remarkable regularity in the periodicity 

 of their breeding habits. During their immaturity all the Palolos 



1 Bourne, "The Ctenophora," Treatise on Zoology, vol. ii., London, 1900. 



2 Child, " The Habits and Natural History of Stychostemma," American 

 Naturalist, vol. xxxv., 1901. 



3 Semper, Animal Life, London, 1881. 



