ON ANNELIDA. 53 



zation, the little we do know is essentially owing to Pallas, 

 Some genera have also been treated of by Baron Cuvier and 

 Sir Everard Home, and among the rest the Arenicola, which is 

 one of the largest species in our European seas. 



The body of the chetopoda, in general elongated and ver- 

 miform, is sometimes, however, merely a lengthened oval, the 

 longitudinal diameter not exceeding the transverse more than 

 two or three times, as is the case, for example, in the genus 

 aphrodite. It is also sometimes cylindrical, as in the lum- 

 brici, and some other genera. But the permanent character 

 of the body in this group of animals, is its being divided into 

 a considerable number of rings, segments, or articulations, by 

 transverse furrows, in which the skin, being softer, admits of the 

 movements necessary for locomotion. The number of these 

 segments varies considerably, and in some genera, as, for in- 

 stance, the serpulae, becomes an important character for the dis- 

 tinction of species. The rings too, at least in diameter, are 

 never rigorously alike, but are gradually diminished towards the 

 two extremities of the body. The head is constantly distinct, 

 but it seldom happens that it is composed of a single ring. It 

 is even difficult occasionally to decide where it begins, be- 

 cause it is never separated from the rest by a series of 

 articulations, forming any thing like a neck. It is therefore 

 necessary, in applying the denomination of head, to take its 

 appendages into consideration. 



In the majority of the species, such as the nereides, the am- 

 phinomse, and still more in the lumbrici, not only is there no 

 neck, but it is impossible to find means of separating the 

 trunk into thorax, abdomen, and tail. But such is not alto- 

 gether the case with the serpulte and amphitritae : in those 

 groups a certain number of rings which follow the head are 

 truly different from those which form the rest of the body, and 

 a sort of thoracic region is perfectly distinguishable, and con- 

 sequently an abdominal one. There is no caudal part, how- 



