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 how^ever 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 Dentalia, 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- 



BRANCHIA. 



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 Linnasus under the 

 name of lumhricus 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 ten taenia 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, 



