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 setm 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. Bose. 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. 
