NOTES ON ANODON AND UN 10. 



Plate T. 



By Oswald H. Latter, M.A., Formerly Berkeley Fellow of Owens 

 College, Manchester, 1888, Late Tutor of Keble College, Assistant 

 Master at Charterhouse. 



[From the Proceedings of the Zoological Society of London, 

 January 20, 1891.] 



I. 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 observa- 

 tions have led me to somewhat different conclusions. Yon 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 

 somewhat posterior to the external aperture of the nephridium ; it 

 is slightly anterior to the commencement of the free detached dorsal 

 border of the inner lamella of the internal gill-plate. The ova may 

 be seen through the thin epithelial covering on the dorsal margin of 



D 



