52 MR. O. H. LATTER ON ANODON AND UNIO. [Jan. 20, 



general setse of the body as terminating in a bifid extremity like 

 those of the Tubificidae and some other families of aquatic Oligo- 

 chaeta. On the other hand, the characteristic /-shaped setse of the 

 terrestrial Oligochseta are often found among aquatic genera ; it is 

 the converse that is rare. 



2. Notes on Anodon and Unio. By Oswald H. Latter, 

 M.A., formerly Berkeley Fellow of Owens College, 

 Manchester, 1888, late Tutor of Keble College, Assist- 

 ant Master at Charterhouse. (Communicated by 

 Frank E. Beddard, M.A., Prosector to the Society.) 



[Eeeeived Noyember 13, 1890.] 

 (Plate VII.) 



The following observations have been made from time to time 

 during the last two years while workino- at Anodon and Unio for 

 other purposes. I hare thought it worth while to bring them to- 

 gether and publish them apart from the anatomical and other details 

 which I hope to complete shortly. ^ly investigations were begun 

 in Manchester in 1S88, while I held the Bishop Berkeley Fellowship, 

 and I may take this opportunity of thanking the donor of that 

 emolument for the facilities tiiereby afforded me and also Prof. 

 Milnes Marshall for his kind advice and assistance in many ways. 



I. The Passage of tlie Ova from the Ovary to the External 



Gill-plate. 



[n 1830 von Baer gave in Meckel's ' Archiv,' 1830, pp. 313-352, 

 an account of this process, which has, so far as I can ascertain, been 

 tacitly accepted by all later writers on the subject. My own obser- 

 vations have led me to somewhat different couclusions. Von Baer's 

 account is briefly as follows : — The ova pass along the inner branchial 

 passage, being prevented from falling into the internal gill-space by 

 the labour contractions of the foot ; thence they pass into the cloaca, 

 into which the outer branchial passage also opens. All the muscles 

 of the body are in a state of contraction during the passage of the 

 ova, and furthermore the cloaca is small. In consequence of the 

 muscular contraction the shell is closed and the ova accumulate in 

 the cloaca, a few perhaps being emitted into the water before the 

 closure is complete. The only direction therefore along which the 

 pressure of ova can be relieved is forwards along the outer branchial 

 passage and thus to the external gill-space. It is to be noticed that 

 von Baer does not state that he has observed these phenomena, but 

 merely draws his conclusions from the anatomical relations of the 

 various organs. 



I have myself observed the passage of ova as far as the cloaca. 

 The genital aperture, as is well known, is situated ventral of and 



