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 serpulse, 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 serpulae 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- 
