ON ANNELIDA. 109 



made in a serpentine manner from right to left. Otho Fabri- 

 cius tells us that he has even seen the two extremities moving 

 at once in the same direction. It appears that certain species 

 can swim equally well in this serpentine way, without doubt, 

 by means of the oars, formed by the pencils of setae of their 

 appendages, and especially by the aid of their foliaceous cirri. 



Those which bury themselves in the sand or mud, which 

 doubtless are the unidentated species, appear to do this by 

 the assistance of the tuberculous points with which their pro- 

 boscidian rings are armed. 



We know but little concerning the natural history of the 

 nereides, except what we have received from Otho Fabricius 

 and M. Bosc. It is generally admitted that the species of 

 this genus are found only in the waters of the sea. 



It is equally evident that there are species of nereides in 

 all parts of the world, although they are very far from having 

 been sufficiently studied. From the little that we do know 

 concerning their division, it would appear that a species of 

 each group may exist in the different zones of the globe, but 

 the largest belong to the torrid zones of the two sides of the 

 equator. 



The nereides most usually live in the excavations of littoral 

 rocks, in the hollows of sponges, in certain alcyones, in uni- 

 valve or bivalve shells, in madrepores, in the interstices of 

 the radicles of thalassiophytes, under stones, and in general 

 in all bodies which present fissures more or less profound. 

 There are some which bury themselves in mud or sand, where 

 they excavate a lodge proportional to the dimensions of their 

 body, and sometimes they line this dwelling with a mucous 

 matter issuing from their body in sufficient abundance to 

 construct a tube or sheath. From this they put forth a 

 greater or less portion of their body, but rarely the posterior 

 extremity, so that they may be able to re-enter on the slightest 

 indication of danger. 



