994 SUMMARY OP CURRENT RESEARCHES RELATING TO 



According to tho ono view, the fact whether the single band is a 

 prre-oral or a post-oral one depends upon the position in which tho 

 anus is about to dcvclopc. If the anus developes in such a position 

 that mouth and anus lie upon ono and the same side of the band, tho 

 latter becomes pra>oral ; if, however, the anus developes so that 

 mouth and anus lie upon opposite sides of the band, the band becomes 

 post-oral. If we hold this view, wo must consider any second band, 

 whether prse- or post-oral to arise as a new development. The other 

 view premises that the anus always forms so as to leave the primitive 

 ring or " architroch " post-oral, i. c. between mouth and anus. Con- 

 currently with the development of a prostomium this architroch 

 somewhat changes its position, and the two lateral portions come to 

 lie longitudinally ; these may be supposed to have met in the median 

 dorsal line, and to have coalesced, so as to leave two rings, tho one 

 pr;c-oral (a " cephalotroch "), the other post-oral (a " branchiotroch "). 

 This latter may atrophy, leaving the single pree-oral ring, or it may 

 become further developed and thrown into more or less elaborate folds. 



The existing condition of the trochal disc or velum in tho Eoti- 

 fera seems to the author to bear out the latter view as to the way in 

 which the modifications of the velum may have come about ; further, 

 these results may be well compared with those recently obtained by 

 Selenka in the Sipunculids. The trochal disc in tho Rotifera in its 

 simplest condition forms a single circum-oral ring, as in Microcodon. 

 This simple ring may be thrown into folds, so forming a series of 

 processes standing up around the mouth ; this is the condition in 

 Stephanoceros. There are, however, but few forms presenting this 

 simple condition, and it must be remembered that the evidence for 

 the assumption here made is at present inconclusive. This band may, 

 while remaining single and perfectly continuous, become prolonged 

 around a lobe overhanging the mouth — a prostomium. This con- 

 dition occurs in Philodina ; the two sides of the post-oral ring do not 

 meet dorsally, but are carried up, and are continuous with the row 

 of cilia lining the " wheels." There is thus one continuous ciliated 

 band, a portion which runs up in front of the mouth. This condition 

 corresponds to that of the Auricularian larva. The folding of the 

 band has become already somewhat complicated. We have only to go 

 a slight step further and the prostomial portion of the band becomes 

 separated as a distinct ring, a cephalotroch. We find such a stage in 

 Lacinularia, where both cephalotroch and branchiotroch remain fairly 

 simple in shape. In Melicerta the branchiotroch is becoming thrown 

 into folds. Lastly, we find that in such forms as Brachicmus the 

 cephalotroch becomes first convoluted and then discontinuous, and 

 further it may become so reduced as to be represented only by a few 

 isolated tufts. In such a form as Lindia the branchiotroch has 

 become reduced to be two small patches at the sides of the head. 



Defectiveness of the Eye-spot as a means of generic distinction 

 in the Philodinsea.* — Mr. W. Milne discusses Ehrenberg's sub- 

 division of this family, and concludes that the existence of eye-spots 



* Proc. Phil. Soc. Glasgow, xvii. (1885-6; pp. 134-45 (2 pis.). 



