REPRODUCTION 



21/ 



fertilisation, and develop immediately ; they are often hatched 

 within the tube of the tubicolous species. 



Under certain conditions the unfertilised females produce 

 exclusively smaller eggs, which develop into males. Maupas 1 

 has demonstrated that a rise in temperature to a minimum of 

 26 C. (79 F.) is the efficient factor. But as Bergendal points out, 2 

 the critical temperature probably varies with the antecedent con- 

 ditions of the race, since males occur in Greenland at a very 



Fig. 113. Diglena catellina. (After Weber.) A, Male ; B, the pair in copula; 

 C, female, p, Penis ; te, testis. 



much lower temperature ; and it would seem probable that a 

 temperature approaching that at which the pools habitually dry 

 up is what is necessary for the production of males, as a provision 

 for those fertilised eggs, which, having a hard shell often adorned 

 with prickly prominences, and usually remaining for some time 

 before development, are capable of withstanding drought ; such 

 eggs are termed "winter eggs," but a better term would be 

 " resting eggs " (German, " Dauereier "). 3 



The male organs consist of a testis (Fig. 113, A, te) with acces- 

 sory glands, a large seminal vesicle, and a protrusible or project- 

 ing penis (p). In Notommata and Diglena true intromission 

 at the cloaca (B) has been seen by many observers; but it 



1 C. B. Ac. Sci. cxi. 1890, p. 310 ; cxiii. 1891, p. 388. 



2 Acta Univ. Lund, xxviii. 1891-92. 



[ 3 See, however, Caiman, Natural Science, xiii. 1898, p. 43.] 



