44 



All forms of group A correspond with those of group d witli the exception of Dondersia 

 and Macellomenia; />' and P correspond likewise. 



The question arises now, whether there is any relation between both groups and whether 

 there may be transition-forms between the forms with a thick cuticle, pointed spicula, papillae 

 and a polystichous radula, and forms with a thin cnticle, flat imbricated spicula, a distichous 

 radula, and without papillae. 



Such a transition-form has been describecl in the first place by Pruvot for Stylomenia 

 (10): the radula of this form may have originated from that of Ismenia, whilst the radula ot 

 Stylomenia may have given rise to a form like that of Amphimenia and Proneomenia vagans 

 Kow. & Mar. In that case the integument and the spicula of Amphimenia must have developed 

 from those of Stylomenia; a transition-form between the two is wanting. 



Stvlomenia also presents points of affinity to other forms. In the first place to Dondersia, 

 of which the radula has i purely conical tooth, the distichous character of which can no longer 

 be recognised, whilst Dondersia is in its turn related to Myzomenia and Nematomenia. 



Moreover there is some relation between Stylomenia and Hemimenia on account of the 

 thin cuticle, but more especially on account of the "organes en cordon". A third line of 

 development leads therefore in the direction of Hemimenia and Neomenia. 



A nother group may have developed from Ismenia in the direction of Paramenia. The cuticle 

 of Paramenia is thicker than that of Stylomenia but the papillae are still absent ; the spicula are long 

 and thin but the radula is distichous. A close relation of Paramenia is Proparamenia ; the radula 

 of the latter form is however monoserial. A close observation of this radula will still reveal its origin 

 from a distichous radula (cf. fig. 88); when the teeth of a distichous radula approach each other 

 and broaden whilst the basal ends fuse into each other, a radula like" that of Proparamenia may 

 orisrinate. From the latter form radula-forms like those of Macellomenia and Proneomenia may 

 spring. Pruvot mentions that in forms with a distichous radula the salivary glands open out 

 separately, while in those with a polystichous radula the salivary glands unite before opening out 

 into the pharynx. This view correct at the time he wrote, is no longer of value. Forms with a 

 polystichous radula like Proparamenia, Proneomenia Weberi, Proneomenia longa and Proneomenia 

 thulensis have 2 salivary glands, which remain separated. Pruvot says (10, pag. 486): "on concoit 

 que si les orifices (des glandes salivaires) viennent a se rapprocher, il en sera de mème des deux 

 rangées de crochets comprises entre eux, et quand les orifices se seront fusionnés en un, seul les 

 odontoblastes produiront une bandelette unique". This holds good for forms like Proneomenia 

 vagans, Dondersia and others but it is impossible that e. g. in Proneomenia Weberi it should be 

 because of the salivary glands, merging clearly separated into the pharynx that the polystichous 

 radula has sprung from a distichous radula. The polystichous radula of these forms is far better 

 explained by supposing it to have sprung from the radula-forms of Proparamenia and Paramenia. 

 The presence of the 2 broad radular teeth, which in Proneomenia Weberi and longa are found 

 in the middle of the radula (cf. fig. 17, 28) reminds me of the tooth of Proparamenia and is 

 in favour of his view. krom this circumstance it follows that it would be better perhaps not 

 to class the 2 above mentioned forms with the other species of Proneomenia as 1 genus. 

 Especially as far as the radula of Proneomenia is concerned, our knowledge is still insufficiënt, 



