displaced anteriorly to segment V). Each 

 spermatheca consists of an ampulla and a duct 

 which opens to the exterior through the sper- 

 mathecal pore. Sperm within the spermathecae 

 may be in random masses, or organized into 

 discrete, regularly oriented, usually radially 

 symmetrical, spermatozeugmata. In the genital 

 region of some species the ventral setae may be 

 absent, or modified as genital (penial and sper- 

 mathecal) setae. The latter can differ markedly 

 in size or shape, or both, from the remaining 

 body setae (somatic setae). The form of the 

 genitalia and their segmental position are of 

 paramount systematic importance at all taxo- 

 nomic levels, especially in the microdriles; in 

 many cases, therefore, only sexually mature in- 

 dividuals can be identified with certainty. 



Microdriles are all small worms (usually up 

 to 1.0 mm in diameter and 60 mm long) without 

 a gizzard, and usually with a variable number of 

 setae in each bundle in diflFerent body regions. 

 Hair setae and genital setae are present in some 

 Naididae and Tubificidae, but both are absent 

 in Enchytraeidae: the dorsal chrochets of 

 Naididae are usually referred to as needle setae. 

 Most microdriles have one pair of spermathecae, 

 and male genital ducts which are located in, or 

 open on, the segment just posteriorly to the 

 testes; that is, the vasa deferentia penetrate 

 only one septum (Fig. 2). The clitellum is one 

 cell thick, and in the smaller species it may be 

 inconspicuous and confined to one segment. In 

 the taxonomically important tubificid male gen- 

 italia, the vasa deferentia enter ectodermal atria, 

 each (of the pair) consisting of a layer of inner 

 lining cells, and an outer m.uscular layer cov- 

 ered by peritoneal epithelium. The proximal 

 end of the atrium or atrial duct may terminate 

 at the male pore either simply, or, more usually, 

 as some form of modified penis which consists 

 basically of a deep infolded ring of body wall iso- 

 lating a central cylindrical organ; this, a true 

 iienis, may be surrounded by a thickened layer 

 of cuticle known as the penis sheath. A mass of 

 darkly staining cells, the prostate gland, whose 

 secretions probably provide nutrients and bind- 

 ing fluid for the sperm mass prior to copulation, 

 usually joins the atrium by a stalk composed of 

 elongated processes of the gland cells. In some 

 species, however, the prostate gland is a diffuse 

 layer of cells surrounding the whole, or part, of 



the atrium, and is absent as a discrete organ 

 in at least one species. 



The Megascolecidae, which are more nearly 

 related to the familiar lumbricid earthworms 

 than to the previous three families, are large 

 worms (exceeding 1.5 to 2.0 mm in diameter 

 and 50 mm in length) , usually with a gizzard and 

 with two setae in each bundle. The important 

 diflFerence between the male genitalia of mi- 

 crodriles and megascolecids is that, whereas in 

 microdriles the vasa deferentia pass through 

 only one septum, in megascolecids the vasa de- 

 ferentia always penetrate about six septa before 

 opening to the exterior: in the case of the ma- 

 rine species, two pairs of testes occur in seg- 

 ments X and XI, and one pair of male openings 

 are located on segment XVII or XVIII. Two 

 to four pairs of spermathecal openings are pre- 

 sent, the most posterior of which is located in 

 or near intersegmental furrow 8/9. The clitel- 

 lum is more than one cell thick and begins in 

 the region of the thirteenth segment. 



Ecology 



The marine Oligochaeta are essentially benthic 

 organisms which live within, and feed on, the 

 bottom deposits. Many species are free-burrow- 

 ing animals which feed indiscriminately on the 

 bottom deposits, while others are meiobenthic 

 (interstitial) worms which inhabit the inter- 

 stices of the substrata and feed only on the very 

 small organic particles, or browse off material 

 from the surfaces of the larger particles. Mega- 

 scolecidae and some Naididae, Tubificidae, and 

 Enchytraeidae are free-burrowing, and some 

 Tubificidae, Enchytraeidae, and possibly some 

 Naididae, are meiobenthic. The major require- 

 ments for the survival of oligochaetes in the 

 marine environment, apart from the obvious 

 physiological adaptions, therefore, are the avail- 

 ability of suitable substrata and their ability to 

 compete successfully with other deposit feeders 

 in a given habitat (for example, Polychaeta, 

 Echinodermata, Mollusca, Crustacea). Despite 

 the fact that such situations are frequent from 

 the littoral zone to the abyssal plain, the Oligo- 

 chaeta are only now being recognized as regular 

 components of the marine benthic ecosystem; 

 hence little specific information on their ecology 

 is available. It is known, however, that oligo- 

 chaetes assume a very important role in polluted 



