AMPHIPOD GENUS PARATHEMISTO — BOWMAN 363 



Gammarus new segments are formed by division of the two (or three 

 in older animals) proximal flagellar segments with the result that two 

 (or three) new segments are added at each molt. Segerstrale found 

 that the antennal segments of the male Pontoporeia become greatly 

 lengthened at the last molt; lengthening at this time must also occur 

 in Parathemisto (compare figs. 2a and 3a). 



It is apparent that after an unknown number of instars during 

 which the antennal flagellum remains unisegmental, the male Para- 

 themisto passes through a minimum of three additional instars. 

 According to Mogk (1926), the male Phronima sedentaria (Forskal) 

 passes through five instars with an unsegmented flagellum. Segments 

 first appear in the sixth instar, and the male reaches maturity in the 

 seventh instar. In Phronima atlantica Guerin segmentation is also 

 first apparent in the sixth instar, but the adult condition is not reached 

 until the eighth instar. 



In the first stage after leaving the marsupium, the flagellum of 

 antenna 1 of Gammarus chevreuxi Sexton (Sexton, 1924), G. fasciatus 

 Say (Clemens, 1950), and Pontoporeia affinis Lindstrom (Segerstrale, 

 1937) consists of four segments. By the fourth stage the segments 

 number 8 for G. chevreuxi and 9 for G. fasciatus; and in the fifth post- 

 marsupial instar there are 10 and 9 segments, respectively. On the 

 assumption that the first instar of P. pacijica with segmented flagella 

 corresponds to the fourth or fifth instar of Gammarus, we can postulate 

 that there are 3 or 4 postmarsupial instars of Parathemisto with 

 unsegmented flagellae, or a total of 5 to 6 postmarsupial molts required 

 to attain sexual maturit}*. In gammarideans which have been 

 studied, from 7-14 molts are required (Sexton, 1924, 1928; Roux, 

 1933; Clemens, 1950; Kinne, 1953, 1959). 



Oostegites first appear in the female when she is quitesmall (3.1 mm. in 

 the earliest stage illustrated). At this stage, the oostegites are very 

 small lobes arising mediad to the gills and only about one-third the 

 length of the gills. During subsequent molts the oostegites increase in 

 size, and in the adult they are transparent, membranous and much 

 larger than the gills. No marginal setae are present. Development 

 of the oostegites in P. libellula is identical (Dunbar, 1957). 



The size at which maturity is reached varies widely. As an example, 

 382 females and 491 males from station 702, cruise 1, were examined 

 for maturity, judged in the males by the elongate antennae and in the 

 females by the presence of fully developed oostegites. The results are 

 given in table 1 . 



The males attain maturity at a slightly smaller size than the females, 

 though not necessarily at a younger age. 



Food: Reports of the food habits of P. gavdichavdii and P. libellula 

 have been given by Bigelow (1926, p. 107) and Dunbar (1946) respec- 



