850 MRS. E. W. SEXTON ON AMPIIIPODA [Nov. 23, 



■of the great modification of the chitinous cuticle of the body 

 with age. In the young from the incubatory jjouch, 2'25 mm. 

 long, the dorsum is perfectly smooth, but, under a high powei', a 

 microscopic sensory setule can be seen, inset at ench point where 

 later either a carina or a hump is developed. The first dorsal 

 hump of the perpeon does not develop till the animal has 

 attained a length of 5 mm., and not until two stages later, when 

 it is a little more than half -grown, can the sexes be distinguished. 

 At this stage, 8-9 mm. in length, with three dorsal humps 

 developed, the young female shows the incubatory lamellae just 

 budding as small excrescences, and the young male carries the 

 characteristic masses of sensory filaments on the antennte. At the 

 next stage in the female, 10 mm. length, with four dorsal humps 

 developed, breeding has commenced, the eggs are extruded and 

 carried in a large mass attached to the 5th segment, but the 

 lamellse are not more than half -grown, and only cover about 

 half the egg-mass. In other animals at this same stage, 

 presumably older, measuring 10"5 mm., the incubatory lamellre 

 are fully expanded and closed to form the pouch. All the 

 remaining stages, to the full-grown female with seven well- 

 marked dorsal humps, carry eggs, and all have the lamellae fully 

 expanded. 



Sexual dimorphism is a common feature of the Amphipoda, 

 the second gnathopod being the organ generally affected. One 

 species in the ' Huxley ' collection, Si/mpleustes grancUmanus 

 Ohevreux, displays a very unusual and marked type of di- 

 morphism, the side-plates and perpeopods difiering wiclely in the 

 sexes, as well as both the gnathopods {infra, p. 857). 



An examination of the present material shows that the mouth- 

 oro-ans are practically constant through the various stages, only 

 the number of spines and setae inci'easing with age ; they would 

 appear to be the safest characters on which to base specific 

 distinctions. The antennae and gnathopods, from which such 

 characters are usually drawn, are the parts always the most 

 affected by sex and development. But it is in the sensory 

 equipment of the animal that most change is to be seen after 

 sexvial maturity is reached : the ommatidia increase in number ; 

 the flageUa of the antennae increase in length, those of the male 

 to a far greater degree than those of the female ; the calceoli, 

 sensory filaments ("olfactory cylinders," " Riechzapfen "), &c. 

 develop in both sexes ; and the peduncles of the antennae in the 

 male become covered with masses or thick tufts of sensory 

 filaments or sensory setae. 



There seems to be surprisingly little individual variation in the 

 difi;erent stages, taken under the same conditions, though animals 

 in the same stage of development but captured in different 

 localities show considerable variation in size. I examined over 

 100 individuals of one stage' of Rhachotropis helleri Boeck, from 

 the S.W. of Ireland, taken in one haul, and they diflfered from 

 each other neither in size, nor proportions, nor even in the 



