870 MRS. E. W. SEXTON ON AMPHIPODA [NoV. 23, 



postero-lateral margins firmly serrate. On consulting Prof. Sars 

 and Mr. Tattei'sall on the matter, they most kindly sent me 

 specimens which prove these differences to be due to age. Mr. 

 Tattersall's specimens, about 250 in number, all taken at one 

 station by the ' Helga,' show a very interesting regular series 

 of the stages of development, from the young in the incubatoi-y 

 pouch with the dorsum smooth to the full-grown female with all 

 seven perseon-segments dorsally raised. 



In this species the completion of sexual maturity in the female 

 coincides with the development of the fourth dorsal perseon-hump 

 at 10 mm. length. A previous stage, 8 mm., shows the incubatory 

 lamellae just starting as veiy small glabrous plates, but none of 

 the specimens of this size had the lamellae further expanded, nor 

 carried eggs. Specimens 10 mm. long, with the 1st, 2nd, 3rd, 

 and 7th perteon-humps produced, had the lamellae half-grown, 

 with two or three minute hairs near the apices, and carried large 

 rounded masses of eggs protruding considerably beyond the 

 lamellae and flattened underneath against the body-wall. The 

 eggs, about 60 in number, each measuring -25 mm., are supported 

 on long bulbous stalks, branching from a short central stem by 

 which- the mass is attached to the 5th segment (see fig. 51). 

 Specimens of 10'5 mm. length show the lamellae fully developed, 

 bordered with long hairs and closed to form the pouch. The 

 e<^gs enclosed are separate from each other, fewer in number, and 

 large, some measuring as much as 1 mm. 



Most of the Irish specimens are at this latter stage of develop- 

 ment, answering exactly to the description and figures given by 

 Sars (pi. 150); the Norwegian specimens at this same stage are 

 larger, averaging 12 mm. About 40 of the Irish specimens were 

 larger than the rest, all ovigerous, all with the 1st, 2nd, 3rd, 

 6th, and 7th humps, and the lateral carinte of the 7th segment, 

 and in most cases with the 4th and 5th humps perceptible as 

 slight swellings of the posterior margins. 



Of the 'Huxley' specimens, 18 of the 19 females were full- 

 grown, with all seven dorsal humps produced. The remaining 

 female, of 10 mm. length, had only six humps, that of the 5th 

 segment not being developed. This specimen is younger than 

 the others, as is proved by the fact that the antennae had only 

 10 joints in each flagellum, instead of the 12 joints of the larger 

 animals. 



The males are distinguishable from the females at a glance by 

 the shorter, more compressed perseon, and the long filiform 

 flagella of the antennae. Unfortunately, none of them are full- 

 orown, though sexually mature, as shown by the development of 

 the antennae. Of the Irish specimens, 14 measure 10 mm. In 

 all these the 7th hump is large and the 1st very small, the other 

 segments smooth ; no trace of the lateral carinas on the 7th ; 

 the antennal joints as Sars has figured them, the 4th joint of the 

 peduncle of the inferior antenna being three-quarters the length 

 of tlie 5th. 



