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The Problem of Sex Determination in 

 Dinophilus gyrociliatus. 



By 

 Cresswell Shearer, M.A. 



Clahe College, Cambridge. 



The group of primitive Annelids Dinophilus comprises some eight or 

 nine species. They are remarkable for the fact that some show a 

 well marked sexual dimorphism, in which the male is rudimentary, 

 without any mouth or digestive tract, while in others the sexes are 

 the same size and exhibit no signs of this dimorphism. The group 

 as a class, therefore, is readily divisible into two subdivisions, in one 

 of which all the species are sexually dimorphic, unpigmented, 

 while in the other they are highly pigmented, a bright red, and 

 are sexually monomorphic. The former may be called the Leu- 

 codinophilidae, while the latter may be called the Erythrodino- 

 philidae. The known species, many of which are of doubtful specific 

 value, may be arranged under these two subdivisions, as follows : — 



Leucodiuophilidae 



Erytlii'odiiiophilidae 



1. DinojyJtilus (/yrociliafus, Schmidt, 1857. 



2. „ ConMini, Nelson, 1907. 



3. ,, apatris, Kor.scliolt, 1882. 



4. ., metameroidef', Hallez, 1879. 



5. ,, 'ptj(jmaeus, Verrill, 1892. 



6. ,, vorticoides, Schmidt, 1848. 



7. ,, Gardineri, Moore, 1899. 



8. ,, iccniatus, Harmer, 1889. 



9. „ iii<ia-<, Weldoii, 1886. 



Of the Leucodiuophilidae the first three species, D. gf//vci/iatiis, 

 D. Conklini, and D. (qKitris, are closely related, and are probably one 

 and the same. The form on which the following work has been done 

 is one of these three species, though exactly which of the three 

 I have been unable to decide. T have placed it under the head 

 of D. gyrociliatvs, as this is the oldest of these names. Figures 

 of the male and female drawn to scale, are shown in Fig. 1. 

 It will be seen that the female is very much larger than the 



