THE BREEDING SEASON 9 



considerably longer period than in the case of corals inhabiting 

 temperate waters. 



It may also be noted that, whereas in the Ctenophora of the 

 Mediterranean the breeding season extends throughout the year, in 

 members of the same class in northern seas it only lasts through the 

 summer. 1 \ 



NEMERTEA, ETC. 



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 deposited always during the night, or in 

 darkness, vrhen 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 Polychiet Annelids, known as the Palolo worms, 

 exhibit a quite remarkable regularity in the periodicity of their 

 breeding habits. During their immaturity all the Palolos live in 

 burrows at the bottom of the water. With the attainment of sexual 

 maturity, and under certain peculiar conditions, they swarm out for 



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. 



I A 



