CHAETOPODA 253 



Though the chaetae are not borne on parapodia they are usually 

 divided into two bundles or groups on each side which roughly 

 correspond to the noto- and neuropodia. They may be classified into 

 hair chaetae which are long and fine (dorsal chaetae of Stylaria) and 

 shorter chaetae which are rod-like (Lumbricus) or needle-like. The 

 point of the needle is single- or double-pronged. There is not, however, 

 the great variability found in the Polychaeta. 



Certain main features of the reproductive system (Fig. 188) are 

 the salient characters of the group. Its members are, without excep- 

 tion, hermaphrodite, and with a single possible exception cross- 

 fertilization only is possible. The restriction of the gonads to a few 

 segments occurs also in some sabellids among the Polychaeta and in 

 some archiannelids. The sexual cells are shed into the coelom either 

 into the general coelomic cavity as in the Polychaeta or into special parts 

 of it divided off from the rest {seminal vesicles oi Lumbricus) where they 

 mature. Spermathecae are usually present to contain the spermatozoa 

 received from another worm in copulation. The clitellum is a special 

 glandular development of the epidermis whose principal function is 

 the secretion of the substance of the cocoon and the albuminoid 

 material which nourishes the embryo. It is a secondary sexual 

 character which is only present in the reproductive season in most 

 Oligochaeta, but the earthworms {Lumbricus^ Allolobophora) used in 

 zoological laboratories in this country always possess it. Both the 

 clitellum and the cocoon produced by it are found in the Hirudinea. 

 It may also be mentioned that many oligochaets have special copulatory 

 chaetae, sometimes hooked for grasping the other worm or with a 

 sharp point for piercing it. 



For the purposes of the elementary student it is probably best to 

 recognize that the Oligochaeta contain two well-marked oecological 

 types, the *' earthworm", a larger burrowing terrestrial form, and the 

 aquatic oligochaet which is much smaller and simpler in structure. 

 It is probable that the former type is the more primitive ; the aquatic 

 oligochaet shows many characters which resemble those of the archi- 

 annelids and are most likely due to a process of simplification. The 

 reasons for the conclusion that the aquatic oligochaets are not the 

 oldest of these groups are given below. 



The Earthworms 



These are divided into a number of families of which the most 

 important are the Lumbricidae, containing Lumbricus and Allolobo- 

 phora^ and the Megascolecidae which is the largest of all. 



The primitive forms in all families resemble Lumbricus in the 

 following characters. There are a large number of segments and each 



