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VII. — A Note on Fertilization and Deposition of Ova in 



Portunus dejjurator. 



By F. Martin Duncan, F.RM.S. F.E.P.S. 



{Read June 20, 1917.) 



Some years ago, and again recently, I had an opportunity of 

 observing a process in connexion with the deposition of ova by one 

 of our British " Fiddler " crabs, Portunus depurator, which I believe 

 is not very generally known, and is of considerable interest. 



The female crab, if captured at the time when she is " in berry," 

 as it is termed by the fishermen, will be seen to carry the masses 

 of ova under the tail, the eggs being attached to specialized pro- 

 cesses, and looking like bunches of grapes. If a healthy female 

 in such condition be kept under observation in a glass tank well 

 supplied with sea-water, she will in the course of a few days or 

 lono-er period deposit the whole of her visible ova in the normal 

 way ; the ova resting on the floor of the tank, and attached to 

 one another in the typical manner. These ova will, of course, 

 duly undergo segmentation, and develop into Protozotea, which will 

 pass through the various stages connected with their metamor- 

 phosis ere reaching the adult stage. Should the female crab at 

 the end of the deposition of this batch of ova be removed to another 

 tank, and kept under further observation, she will probably in the 

 course of a week or ten days be found to have deposited a fresh 

 quantity of ova, which, however, are not attached to each other, but 

 are scattered like grains of sand upon the floor of the tank. These 

 ova, although there has been no intercourse with a male crab since 

 the first batch was deposited, will also undergo segmentation and 

 f^ive rise to Protozosea which complete their metamorphosis. Their 

 fertilization, however, has not been due to the presence of free- 

 swimming spermatozoa in the sea-water, as appears to have been 

 hastily surmised by some observers. This can be proved, and the 

 true significance of this second deposition of ova better appre- 

 ciated, by carefully filtering the sea-water through two or more 

 thicknesses of filter-paper, so as to preclude all possibility of 

 free-swimming spermatozoa being present in the second tank 

 when the female crab is placed therein. 



It is, I believe, a well-established though not very generally 

 known fact, that one impregnation of the female crab suffices for 



