AMPHIPOD GENUS PARATHEMISTO BOWMAN 361 



this finding is not necessarily true for P. pacifica. On the contrary, 

 examination of the ovaries of females with eggs or young in the brood 

 pouch provides strong evidence that more than one generation of 

 offspring is produced by the individual female. The ovaries can be 

 readily seen through the transparent ventral body wall of the pereon; 

 they lie on either side of the posterior midgut and anterior hindgut, 

 above and in close association with the digestive diverticulae. In 

 the ovigerous females that I examined, I saw new groups of eggs 

 developing in the ovaries. The presence of a second group of eggs 

 indicated that another clutch of eggs would be laid. 



The above evidence indicates that the individual P. pacifica, in con- 

 trast to P. libelluia, produces at least two and perhaps more sets of 

 offspring during its lifetime. It is therefore possible that the July- 

 breeding females are the older and larger members of the generation 

 sampled in March, and that they have already produced one or 

 more litters. 



3. Because of a seasonal difference in vertical distribution, larger 

 specimens, which lived in deeper water in March, were within the 

 range of the nets in July. Earlier in this paper it was deduced from 

 a comparison of da}* and night hauls that the Parathemisto population 

 lives nearer the surface in July than in March or September. This 

 habit would account for the larger average length as well as the larger 

 numbers of the specimens collected in July. It is well established 

 that in most, but not all zooplankters, the adults live at greater depths 

 than the juveniles, and adults from deeper water average larger than 

 those nearer the surface. Bogorov (1940) found that the young of P. 

 abytssorum in the Barents Sea prefer the upper water layers, but accord- 

 ing to Dunbar (1946), P. libelluia has a marked phototropism and sta3 T s 

 near the surface, especially on sunny days. In the Gulf of Maine the 

 young P. gaudichaudii are most numerous near the surface, while 

 adults occur mainly in net hauls made below the surface (Bigelow, 

 1926, p. 163). 



4. The larger individuals collected in July lived in colder waters 

 to the north in March and were carried south by the California 

 Current. This hypothesis would not account for the smaller size of 

 the November specimens, which would have to belong to a different 

 generation. The California Current is slow and meandering, and has 

 numerous eddies. It seems unlikely that any considerable segment 

 of the Parathemisto population that inhabits the California Current 

 is unable to maintain itself latitudinally against the southward flow. 

 I regard this explanation as highly improbable. 



It is improbable that any of the above explanations can, by itself, 

 fully account for the observed seasonal variation in size of P. pacifica. 

 The cruises from which I have studied material are too widely sepa- 



