80 HALF AN HOUR WITH SEA-WORMS. 



tion will detect that these are not jointed, as they 

 are in insects and crustaceans. The sexes are some- 

 times distinct, and at others united in the same 

 individual. This arrangement, however, is exceed- 

 ingly variable, for what is called " gemmation "- 

 that is, the budding of new individuals is also 

 carried on as a means of increasing the species. 

 Singularly enough, there is a kind of " alternation 

 of generation " exemplified here, similar to that so 

 well known in the natural history of the plant-lice 

 (aphides). For instance, a worm that has been 

 produced by " gemmation " will often produce 

 young that will lay eggs; whilst the individuals 

 resulting from the latter can only bring forth 

 " buds " or " gemmules." 



The Nereis margaritacea, or " Sea-centipede," is a 

 sea-worm that well deserves its specific name of 

 " pearly," as every one will confess who has seen its 

 changeable metallic lustre, and its opalescent tints 

 beneath. Generally, it ranges from brown to dark 

 green in the colour of its upper side, subject to the 

 lustrous variations above mentioned. This worm is 

 one of the finest and commonest of the species which 

 haunt our shores. Through the light brown of the 

 back you can see the blood-vessel, or " dorsal heart," 

 contracting at intervals of a few^seconds, and then 

 dilating. It is not at all uncommon for this species 

 to take up its head-quarters in an empty univalve, 

 which it sometimes shares in company with the 

 Hermit Crab. Its two eyes and proboscis are plainly 



