212 DISCOVERY REPORTS 



With the pelagic habit are correlated certain structural modifications of the third 

 maxillipede, and these, as Matthews (1932) has shown, may in certain circumstances be 

 retained until, or almost until, the attainment of sexual maturity. 



Press of work in the field allowed but a cursory examination of fresh material to be 

 made, and although many samples were preserved some hauls were discarded after 

 only identifying, counting, sexing and examining for parasites. In the very large hauls 

 of M. subrugosa which were occasionally taken only fractional samples could be handled, 

 the rest being indiscriminately tossed overboard. The material, large as it is, was ob- 

 tained incidentally in a scheme much larger in scope than the subject of this report, and 

 it thus lacks much of the value of a collection planned with the object of obtaining in- 

 formation regarding the life histories of the two species. Only a few questions are 

 solved in the following report, and the suggestions put forward must wait upon sub- 

 sequent more searching investigations carried out with closer attention to the living 

 animal. 



Mention must be made here of a small but useful collection of post-larval M. gregaria 

 from Otago Harbour, New Zealand, forwarded by the late Mr G. M. Thompson to Mr 

 L. H. Matthews, through whose courtesy it came into the writer's hands. 



LARVAL DEVELOPMENT 



Larval forms of the genus Mimida have been described by Sars (1890), Stephensen 

 (1913), WilHamson (1915), Stebbing (1919), Webb (1921), Gurney (1924), and Lebour 

 (1930). Sars, Webb and Lebour describe the larva of M. banffica, Stephensen the 

 larva of M. tenidmana, whilst Williamson deals with both these northern forms: 

 Stebbing and Gurney are concerned with southern hemisphere forms. Stebbing 

 describes and figures what purports to be a larval stage of M. gregaria collected by 

 Vallentin at the Falkland Islands, but unfortunately the description is scanty and the 

 figures almost useless. Gurney deals with a form taken by the 'Terra Nova' in New 

 Zealand waters. Young (1925) mentions the hatching of larvae from the eggs of M. 

 gregaria by Anderton, but only gives the hatching dates and no description. 



The larval forms of Mimida now to be discussed were collected in tow-nets and other 

 nets during the trawling survey of 193 1-2. In a series of forty-three oblique hauls with 

 aim. tow-net, taken between September 1931 and April 1932, at depths of from 139 to 

 29 m. to the surface, fourteen contained larval forms and three early post-larval forms. 

 The larval forms were taken in the months September to November and in February, 

 but the early post-larvae were limited to January. Hauls in which larvae were not 

 present were made in every month from September to April, except December. 



Graphs showing the percentage of berried females to the total number of females in 

 the monthly catches of M. gregaria and M. subrugosa have been prepared in the hope 

 that the spawning periods would be indicated. The graph for M. gregaria (Fig. i) shows 

 a rapid descent from a high figure in October to zero in November, and this may be 

 taken as indicating the shedding of larvae into the plankton. This spawning time 

 corresponds with September and October given by Young (1925) as the spawning date 



