ARCHIANNELIDA. 



455 



-ff 



by the intestine, which is dilated segmentally. Coelomic septa and dorsal and 

 ventral mesenteries are present. Of the vascular system a dorsal vessel in the 

 dorsal mesentery has been detected. 



The sexes are separate, and the gonads are found in 

 all segments behind the fourteenth. The germ -cells 

 are developed from paired patches of the coelomic 

 epithelium on the posterior faces of the septa, and are 

 dehisced into the body-cavity, where they ripen. They 

 escape by the nephridia of the part of the body in 

 which they are contained. 



The nephridia are simple tubes opening externally 

 close to the bundles of setae, and internally into the 

 preceding segment. In the male the nephridia of the 

 testicular segments are swollen near their openings, 

 and their terminal parts pass into small papillae which 

 project on the surface as small penes. These penes can 

 be retracted into penial sheaths. In the female there 

 are, in the ovarian segments, receptacula seminis ; they 

 are yellowish vesicles lying in the central chamber of 

 the body-cavity, and passing into a duct which traverses 

 the oblique muscles and the lateral chamber to open 

 on the ventral surface. 



Histriodrilus* (Histrio'bdella van Ben.) formerly 

 classified with the leeches, is said to belong to the 

 Archiannelida. It is a small worm (1'4 mm.) parasitic 

 on the lobster, the eggs of which it devours. It has a 

 distinct cephalic region, and an externally segmented 

 body without setae. There is a pair of limbs on the 

 head, and another on the last segment. There are 

 cerebral ganglia and two ventral nerve cords which 

 are continuous with the epidermis and united in each 

 segment to form a ganglion ; there are about eight 

 segments. There is a muscular sub-oesophageal sac 

 armed with three chitinous teeth, and the alimentary 

 canal is ciliated. There are four or five pairs of 

 nephridia, the internal openings of which have not 

 been satisfactorily made out. The sexes are separate, 

 and the generative organs complicated. The ovaries 

 and testes are coelomic. A vascular system has not 

 been observed. The body-cavity is present, and there 

 are dorsal and ventral mesenteries. There are four 

 longitudinal bands of muscular fibres in the body-wall. 



Aeolosoma\ Elirb., often included amongst the Oligo- 

 chaeta as the sole member of the sub-order Aphaneura, 

 must be placed w y ith the forms of uncertain position. 

 It is a small (1-10 mm.), fresh-water transparent worm with a loose and 

 uncertain outline. The prestomium is ciliated ventrally, and bears a pair of 

 ciliated pits. The segmentation is ill-marked. There are four bundles of hair- 

 like setae on each segment behind the peristomium. The ectoderm contains 



FIG. 361. d, The young 

 Polygordius ; G cerebral 

 ganglion ; Wg ciliated 

 pit; Dalimentary canal. 



* Foettinger, Arch. JBioL, 5, 1884, p. 435. 



t Beddard, Monograph of Oligochaeta, Oxford, 1895. 



