BiT WILLIAM A. IIASWELL, M.A., B.SC. 267 



Eislg^'' has recently xniblished an interesting and thorough 

 account of what ho calls a swimming-bladder-like organ in the 

 HesionidcE and Syllidea. In his account of the homologous structures 

 in other groups he makes no allusion to the cceca of the Aphro- 

 ditacea ; but the closeness of the correspondence between the 

 latter and the organs which he describes leaves no room for 

 doubt that they are homologous, and to some extent, analogous 

 structures. Eisigf regards the principal function of the 

 *' swimming-bladder " as being to store up the 0x3' gen sejoarated 

 from the sea-water by the intestine and to serve it out as required • 

 The forms in which he regards this intestinal respiration, with the 

 presence of air-bladders as storehouses for the oxygen, as being 

 best marked are also, he points out, forms in which there are no 

 branchice ; they have, however, a specially well-developed vascular 

 system, and in this respect present a contrast to most Aphroditea. 



The chief circulatory medium in the Apliroditea is the peri- 

 visceral fluid — the pseudohoomal system being but slightly 

 developed. The circulation of the former is very actively kept 

 up by the energetic action of the numerous long cilia in the cavities 

 of the parapodia. This active circulation would seem to show 

 that in this family the perivisceral fluid serves not only as a 

 passive supporting '' connective fluid," but is the chief medium 

 through which the gaseous interchanges involved in respiration 

 are brought about ; and this view of its function is strengthened 

 by the fact that gills in the sense of delicate processes of the 

 body- wall supplied by vessels of the pseudoha^mal system are, 

 (except perhaps in Sigallon), entirely absent. The tubercles 

 which bear the scales, together with certain elevations which 

 occur in some genera in a corresponding position on the segments 

 which do not bear scales, have not uncommonly received the name 



* " Ueber das Yortoinmen eiues scliwimmblasenahnlichen Organs bei 

 Anneliden. Mittchilungen aus der Zoologisclieii Station zu Neapel, '•• Band 

 JI., pp. 225-304, pis. xii.-xv., (1881). 



t L. c, p. 2SG. 



