BRITISH NEMERTEANS, AND SOME NEW BRITISH ANNELIDS. 429 



Plate VII. 



1. Stylet-region of the proboscis of 0. gracilis. x 210 diameters. 



2. Stylet-region of the proboscis of 0. purpurea. x 210 diameters. 



3. Stylet-region of the proboscis of pulchra. x 90 diameters. 



4. Extremity of the posterior region of the proboscis of 0. alba distended with fluid ; a, a group 

 of the peculiar dancing granules. x 90 diameters. 



5. Stylet-region of the proboscis of Polia involuta, Van Beneden. x 700 diameters. 



6. Stylet-region of the proboscis of a young 0. alba, illustrating the first appearance of the 



stylets, and the development of the parts. The organ is drawn as it bulged from a wound 

 in the body-wall of the animal. x 700 diameters. 

 7- Fragment of the oesophagus from a living animal; a, inner edge of ciliated fold; b, sulcus 

 between two folds. x 350 diameters. 



8. Eye of Ommatoplea pulchra. x 210 diameters. 



9. Central stylet and portion of basal apparatus in a large 0. gracilis. x 350 diameters. 



... 10. Transverse section of an everted proboscis in a small specimen of 0. pulchra. The papillose 

 mucous surface has been injured in the manipulation. x 90 diameters. 



... 11. Nerve-cells from a cephalic ganglion of 0. alba. x 400 diameters. 



... 12. Portion of a sperm-sac in Tetrastemma varicolor , showing a streaked and granular aspect, 

 from the varying nature of the contents. x 350 diameters. 



Plate VIII. 



Fig. 1. Aspect of the developing proboscis in 0. melanocephala, about the fifth day after the removal 

 of the original organ. x 55 diameters. 



2. Stylet-region of a developing proboscis in the same species ; /, canal, which by-and-by is 

 occupied by the central stylet. The organ is contracted. x 350 diameters. 



3. Anterior region of Tetrastemma alga, showing the arrangement of the digestive system. 

 Enlarged. 



4. Termination of the posterior chamber of the proboscis (C) in 0. alba, with muscular ribands, 

 x 210 diameters. 



5. Head and anterior portion of Polia involuta, V. Ben. ; f, powerful transverse band of fibres 

 which retains the posterior part of the oesophagus in situ. x 180 diameters. 



6. The central, (a) and lateral stylets (b) from a young 0. alba, on the first appearance of the 

 former. x 700 diameters. 



7. Cephalic ganglia of Tetrastemma varicolor. x 210 diameters. 



8. Unimpregnated ovum of 0. alba ; a, outer coat ; b, inner coat ; c, vitellus ; cl, " micropyle," 

 or cicatrix-like arrangement. x 90 diameters. 



9. Ovum of 0. gracilis after impregnation; a, outer coat; /pinner coat; c, vitellus. x 90 



diameters. 

 .. 10. The inner coat and vitellus of an ovum (of 0. gracilis) at the same stage of development, 



with the relations of the spermatozoa. x 210 diameters. 

 .. 11. Ovum of 0. alba, just before the extrusion of the embryo. x 90 diameters. 

 .. 12. Spermatozoa of Tetrastemma vermiculus. x 1000 diameters. 

 .. 13. Spermatozoa of 0. alba. x 800 diameters. 

 .. 14. Spermatozoa of T. variegatum. x 400 diameters. 



Plate IX. 



ig. 1. Young 0. alba, on extrusion from the egg, somewhat compressed. x 55 diameters. 



2. Young 0. alba eight days older than the preceding; b, stylet-region; c, point where the 



posterior chamber of the proboscis becomes lost, after curving forwards, x 90 diameters. 



3. Structure of the stylet- and reservoir-regions in O. alba. Considerably magnified. 



4. View of the cutis in a living specimen of Borlasia olivacea as a transparent object. x 210 



diameters. 



5. Streaked arrangement of the cutis of B. olivacea, from the dorsum. x 210 diameters. 



6. Pigment-cells from the anterior part of the dorsum of the same species, x 350 diameters. 



7. View of the skin of a living Meckelia annulata. x 350 diameters. 



8. Spermatozoa of Ommatoplea gracilis. x 700 diameters. 



9. Spermatozoa of Polia involuta. x 950 diameters. 



VOL. XXV. PART II. 5 R 



