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Show 52 MR. O. H. LATTER ON ANODON AND UNIO. [Jan. 20, general setae of the body as terminating in a bifid extremity those of the Tubificidae and some other families of aquatic Oligo-chaeta. On the other hand, the characteristic/-shaped setae of the terrestrial Oligochaeta 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, Assistant Master at Charterhouse. (Communicated by F R A N K E. B E D D A R D , M.A., Prosector to the Society.) [Eeceived November 13, 1890.] (Plate VII.) The following observations have been made from time to time during the last two years while working at Anodon and Unio for other purposes. I have thought it worth while to bring them together and publish them apart from the anatomical and other details which I hope to complete shortly. My investigations were begun in Manchester in 1888, while I held the Bishop Berkeley Fellowship, and I may take this opportunity of thanking the donor of that emolument for the facilities thereby afforded me and also Prof. Milnes Marshall for his kind advice and assistance in many ways. L The Passage of the Ova from the Ovary to the External Gill-plate. In 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 observations have led me to somewhat different conclusions. 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 |
