286 THE INVERTEBRATA 



Order OLIGOCHAETA 



Chaetopoda, nearly all land and freshwater forms, with a compara- 

 tively small number of chaetae, not situated on parapodia, with pro- 

 stomium distinct but usually without appendages ; always hermaph- 

 rodite, the male and female gonads being few in number (one or 

 two pairs), situated in fixed segments of the anterior region, the male 

 always anterior to the female; with special genital ducts (coelomo- 

 ducts) opening by funnels into the coelom, spermathecae , and a 

 clitellum present at sexual maturity ; with reproduction by copulation 

 and cross-fertilization; eggs being laid in a cocoon, developing 

 directly without a larval stage. 



In addition the pharynx is not eversible and pharyngeal teeth (such 

 as frequently occur in the Polychaeta) are absent, except in one small 

 family, the Branchiobdellidae, which have ectoparasitic habits similar 

 to the leeches and resemble them in some particulars of structure. 



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, how- 

 ever, the great variety found in the Polychaeta. 



Certain main features of the reproductive system (Fig. 199) are the 

 salient characters of the group. Its members are, without exception, 

 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 of 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 earth- 

 worms [Lumbricus^ Allolobophora) used in zoological laboratories in 

 this country always possess it. Both the clitellum and the cocoon pro- 

 duced 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 



