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TRANSACTIONS OP SECTION D< 



work with the Echinoids found at Plymouth, using the later and more definite 

 larval characters as criteria. The pluteus of Echinus acutus at about a month 

 old has both anterior and posterior ciliated epaulettes, whereas that of E. 

 miliaris lacks the posterior epaulettes. On the other hand, E. miliaris pos- 

 sesses two pairs of green pigment masses in the anterior epaulettes, which are 

 absent from E. acutus. Shearer and de Morgan have found that when these 

 forms are crossed the characters of the hybrid larva; are always the same as 

 those of the mother. 



The writer crossed E. acutus $ xE. miliaris ? and E. miliaris 6* x E. acutus ? 

 in sea-water, the OH ion concentration of which had been raised by NaOH and 

 lowered by HC1 and by acetic acid. As soon as the blastulae swam to the surface 

 they were transferred to normal sea-water. A large number of skeletons were 

 drawn, no selection being made except that misshapen larvae were omitted. Of 

 the latter there were always more than when the crosses were made in normal 

 water. The skeleton drawings showed that there had been no effect on the 

 inheritance. At a month old and later a large number of plutel were examined 

 and in all cases they had wholly maternal epaulettes and pigment masses. 



If Tennent's result be true it is of importance to determine whether it is of 

 general application. The failure to bring it about at Plymouth in no way 

 disproves his case, since we start here on a different basis. He started with a 

 dominance of one species over another, whereas at Plymouth there is a maternal 

 dominance. In the latter case, however, an alteration in the concentration of 

 OH ions in the external medium has no effect on the inheritance. 



21. The Problem of Sex Determination in Dinophilus gyrociliatus. 

 By C. Shearer, M.A. 



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 

 dnnorphism, in which the male is rudimentary, without any mouth or digestive 

 system, 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 sub- 

 divisions, in one of which all the species are sexually dimorphic, unpigmented, 

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

 colour, and sexually monomorphic. The former may be called the Leuco- 

 dinophilidie, while the latter may be called the Erythrodinophilidse. The known 

 species, many of which are of very doubtful specific value, may be arranged 

 under these two subdivisions as follows : — 



Of the Leucodinophilidas the first three species, D. gyrociliatus, D. Conklini, and 

 D. apatris, are closely related, and are probably one and the same species, and 

 the form on which the following work has been done is one of them, though 

 exactly which of the three I have been unable to decide. As the oldest of 

 these names is D. gyrociliatus, I have placed my species under this heading. It 

 was obtained some three years ago from some sandy material collected in 

 Plymouth Sound. I introduced it subsequently into the tanks of the Plymouth 

 Laboratory, where it has since established itself and breeds. 



Korschelt was the first to point out that a marked sexual dimorphism is 

 present in D. apatris, where the male is small and rudimentary. He also 

 observed that the female laid two kinds of eggs, some of which were small 

 and are destined to give rise to the rudimentary males, while others almost six 

 times the size of the small ones and also more numerous, are to give rise to the 

 large females. Here was apparently a clear case in which we get sex deter- 



