566 F. W. GAMBLE AND J. H. ASHWORTH. 



The ueplirostomes of the three posterior nephridia are much smaller than 

 those of the anterior nephridia. X 70. 



Fig. 51. — Longitudinal section of the proximal part of one of the ovarian 

 processes of A. ecaudata. On the left a portion of the gonidial vessel is 

 seen, a branch of which traverses the axis of the process. The wall of the 

 blood-vessel, which is formed of a very thin layer of cells well seen on the left, 

 IS covered by a layer of cells produced by divisions of the ccelomic epithelium. 

 From these the ova are differentiated near the vessel, and recede from it as 

 they increase in size. The ova are covered by a layer of unaltered ccelomic 

 epithelium. The nuclei of the ova gradually increase in size and become 

 vesicular (germinal vesicle. Germ. V.). The germinal vesicle contains a small 

 amount of chromatin in the form of a network, and usually a well-marked 

 nucleolus {Nucl.). Yolk granules are deposited in the protoplasm, but the 

 peripheral layer of protoplasm in a ripe ovum is almost free from granules 

 (see the large ovum near the middle of the figure). Tiie ripe ova have a thick 

 conspicuous vitelline membrane, x 420. 



Fig. 52. — One of the ovarian processes of a medium-sized female specimen 

 of A. ecaudata, showing the blood-vessels. At first the process contains only 

 one axial blood-vessel (as in the preceding figure), but later more vessels 

 penetrate into the process and become connected as shown. X 12. 



Fig. 53. — A portion of a disseclion of A. Grubii from Plymouth (Jan., 

 1899). The second and third nephridia of the right side with their gonads 

 {^Gen. Sir.) and the surrounding structures are shown. The dorsal lips 

 {Nphm. D.) only of the nephrostomes are visible. The second nephridium 

 receives blood from the dorsal longitudinal vessel (Z*. Z. V.). The afferent 

 vessel of the third nephridium is a branch of the ventral vessel. It is accom- 

 panied by a thin band of connective tissue, which spreads out fan-wise and 

 connects the nephridium to the body-wall in the region of the second groove 

 behind the cha:tigerous annulus, i. e. at the boundary of the segment. Tliis is 

 therefore the representative of the septum of the segment, x 4. 



Fig. 54. — A portion of a dissection of A. Grubii from Naples, showing 



the last (fifth) nephridium of the right side with a rudimentary nepiirostome 



{Bud. Nphm.) on the septum in the following segment. The dorsal lip of the 



ephrostome, the gonad, blood-vessels, seta, muscles, and rudimentary septa 



are also shown. X 8. 



(Fig. 55 is omitted.) 



PLATE 27. 



Fig. 56. — The anterior end of Arenicola Claparedii seen from above, 

 with the top of the prostomium sliced off horizontally. The ganglionic por- 

 tion of the brain is dotted, the fibrous matter tinted. The course of the 

 nuchal trroove is clearly shown. The point marked .r, .r is the enlarged 



