ON ANNELIDA. 
91 
very probably is branchial or vascular. It is contained in a 
calcareous tube, tolerably thick, solid, slightly arched, open at 
the two extremities, smooth, striated, or even polygonous at 
its superficies. 
We have very few details, indeed, concerning the manners 
and habits of these animals. It is nevertheless very pro- 
bable, that they live sunk perpendicularly in the sand or ooze, 
into which they can no doubt penetrate more or less. But it 
seems, at least, very doubtful that they can change place, 
transporting their tube along with them. The species of this 
genus are however very common on the sandy coasts of the 
seas of warm climates, and even on those of the Mediterra- 
nean. It appears that they were equally abundant in the 
ancient sea, for many of them are to be found in the fossil 
state. 
We are acquainted with a great number of tubes belonging 
to species of this genus, which are named Dent alia, in con- 
sequence of their resemblance to the tusks of the elephant, 
and which are divided into smooth, or striate, angular, or 
polygonous. 
We come now to the second order of annelida, the Dorsi- 
B RANCHI A. 
Arenicola is a genus of marine worms established by 
Lamarck, and which appears even yet to comprehend but a 
single species, which was designated by Linnaeus under the 
name of lumbricus marinus, and by Pallas under that of 
nereis lumbricoides , while the animal is, in fact, neither a 
lumbricus nor a nereis. The generic characters are the want 
of tentacula and jaws, and the possession of gills only in the 
middle of the body, the two extremities being destitute of 
them. This worm is about eight or ten inches long, and a 
little thicker in the middle, than at the two ends. Its skin is 
marked with a multitude of annular wrinkles. For every 
five there is one bulkier, and more projecting than the others, 
