Metamorphosis and Life-history of Gnathia maxillaris. 473 



liaving the thorax completely segmeuted. These larvae i)laiiily must 

 have been deiived from the Praniza form, because the normal seg- 

 mented larvae are iueai)able of grovvth until they assume the parasitic 

 mode of life: the ouly hypothesis that accounts for the existence of 

 thcse forms is that they are derived from Pranizae which liave beeu 

 brushed off their hosts at a period when they could not perform the 

 tìnal metamorphosis into the adult. Tliis hypothesis is substantially 

 contirmed by tvvo Pranizae measuriug 4,5 mm in my colleetion whicli 

 are aboiit to undergo an ecdysis, and underneath their cutiele can 

 be seen the cutiele of a giant segmented larva. 



We will now turn to the Variation in size of the adults. The 

 adult mal es vary in size from 1 — 8 mm and the adult females from 

 1 — 7 mm. This is an immense range of Variation, and it will be at 

 once suggested that these sizes do not represent the definitive adult 

 size of the animals but are differeut stages in growtli. But this 

 argument cannot apply to the females which produce broods and die 

 when they measure anything from 1 — 7 mm, and as a matter of fact 

 it applies just as little to the males, because I have reared adult 

 males from Pranizae varying in size from 2 — 7 mm; so that it is 

 clear that the size of the adult male is dependent on the size of 

 the Prauiza that undergoes metamorphosis. Nor is it at ali likely 

 that the adult males go on growiug to any appreciable extent after 

 they have assumed the adult form, because firstly their mouth-parts 

 are adapted merely for causing a current in the water and possibly 

 driving small Diatoms and Infusoria into the mouth, and secondly 

 the gut ends blindly. It is therefore an absurd supposition that 

 animals with these means of nutrition can go on growing to three 

 or four times their originai bulk, especially when the differeuce in 

 size of the adults has been proved in numerous cases to be due to 

 the great ditì'erences in size of the Pranizae which undergo meta- 

 morphosis. Furthermore no one has ever seen an adult male ex- 

 uviate. 



In table 1 is presented a curve which shows the frequency per 

 cent with which the various sizes occur in 465 males. The measure- 

 ment in millimeters was taken from the anterior level of the eyes 

 to the base of the telson. 



