Dr. Johnston on the British Nereides. 293 



size ; in the first segment not being larger than the others ; 

 in the jaws having ten serratures ; and in the greater elonga- 

 tion of the tentacular cirri. 



The changes which the Annelides pass through, from the 

 egg state to their maturity, have not been traced by any one, 

 and the general belief appears to be that none of the class un- 

 dergoes any metamorphosis, proceeding from the egg with all 

 the characters and lineaments of the parents. I have no di- 

 rect observation to oppose to this belief, which, however, I 

 have been led to think is questionable. In Plate VI. figure 2. 

 represents what seems to me to be the young of a Nereis, 

 probably of N. pelagica, and the differences between it and the 

 adult are not inconsiderable. The tentacula and tentacular 

 cirri, it will be observed, are wanting, while the head is large 

 and well developed ; and there is a pair of large clavate organs 

 at its junction with the first segment, by the aid of which the 

 little creature appears to move through the water, for in this 

 stage of its life it is an excellent swimmer as well as a swift 

 creeper. There is a prominence at the base of each, perhaps 

 the buds of future tentacular cirri ; and here we observe un- 

 derneath some minute ciliated organ, by whose play a current 

 of water is driven violently along the sides. By the play of 

 other ciliated organs at the tail, similar currents are there 

 created and kept up ; and the whole process forcibly recalls to 

 memory the mechanism by which respiration is carried on in 

 many of the edriophthalmous Crustacea. The eyes are in a 

 straight line, and not less distinct than in the adult. The feet 

 are well developed, bizamous : the caudal segment rounded, 

 of a dark colour, and ciliated all round. I found these sup- 

 posed young in the beginning of September amongst Confervoe : 

 they were about two lines in length, and very active. 



The luminous animalcule sometimes to be seen on the shells 

 of oysters, and delineated by Baker, e Employm. for the 

 Micros/ p. 399. pi. 15 A.) seems to be the same worm, a little 

 further advanced. The tentacula are now developed, as well 

 as a single pair of the tentacular cirri. " This little insect, 

 says Baker, " can emit or conceal its light ; and sometimes its 

 lustre is so bright as to be discoverable even in open daylight, 

 especially on being touched or disturbed. Its light is bluish 

 like that of the glowworm, or a spark of burning brimstone/' 



