Chap. HI. GENUS IDYIA. §71' 



apart, in comparison to their size, than the adults, and the ciHa themselves much 

 longer, and fewer in number, so that the motions of the young are much more 

 energetic, and quicker, than those of the adults. This will appear very natural, 

 when it is considered that the smaller individuals have the longer oars to move 

 with, and the older and bulkier individuals the shorter locomotive apparatus, ac- 

 cording to their size. 



But though there are true Beroids in which the locomotive flappers are ever 

 enlarging with age in the direction of the mouth, there are others which, even 

 in their adult condition, have their rows of locomotive flappers limited to as short 

 a range as the young of the former, and, on account of some other peculiaiities, 

 may be considered as a distinct genus. I shall take an 02)portunity hereafter to 

 describe the species of this type which I have observed. The genus Pandora Esch. 

 has such limited rows of locomotive flappers ; but it differs further in having the 

 abactinal part of the spherosome broader and more rounded, the vertical axis 

 shorter than any of the other true Beroids, and the interambulacra so much devel- 

 oped, as, in their contraction, to overlap the locomotive flappers. As for the genus 

 described by Lesson under the name of Cydalisin, I agree with Gegenbaur that it 

 is founded upon characters which have no generic value, and yet I am not inclined 

 to go as far as he does, in uniting all true Beroids in one single genus ; for on 

 comparmg the descriptions and figures published by Milne-Edwards of Beroe Forskah, 

 I find that the species of our coast never assumes that sugar-loaf form which Milne- 

 Edwards represents, but exhibits always rounded outlines on its abactinal side. 

 There must, therefore, be some marked structural difference in the abactinal area 

 of our species and that of the Mediterranean. Accordingly, instead of uniting into 

 one genus all the Beroids which in their adult state have rows of locomotive flap- 

 pers extending to near the margin of the mouth, I would retain the distinction 

 hitherto made between Beroe proper and Idyia, and refer to the genus Beroe those 

 species which resemble the Beroe Forskali, and to the genus Idyia those which 

 resemble the Beroe Cucumis of Sars and the species of our coast. 



Thus circumscribed, the genus Idyia may be characterized by the inequality of 

 its anterior and posterior spheromeres, compared to the lateral ones ; and though 

 this inequality is but slight, it is no doubt sufficient to prevent the abactinal side 

 of the body from being raised into a projecting cone. The structure is this. On 

 their abactinal side the lateral spheromeres are bulging while they converge towards 

 the central eye-speck, whereas the anterior and the posterior spheromeres curve 

 evenly towards the same point. The consequence of this inequality is, that, how- 

 ever miTch the centre itself may be projected, the anterior and the posterior 

 spheromeres act as bridles upon the lateral ones to prevent the centre from rising 

 into the shape of a cone ; while in a state of comparative rest, the abactinal area 



