ANATOMY AND CLASSIFICATION OF THE ARENIGOLIDiE. 563 



the heart.. Sections of tlie iiwaginated processes show the central core of the 

 peritoneal cells surrounded by the muscle layer (the endothelium cannot be 

 shown on this scale of magnification). The space occupied by the blood is 

 coloured red. x S6. 



Fig. 39. — Longitudinal section of the heart of a specimen of A. Grubii 

 225 mm. long from Port St. Mary. The wall of the heart is much thicker in 

 this older specimen, chiefly owing to the greater development of the muscle 

 layer. The invaginations of the wall are very numerous and comjilex, filling 

 up the greater part of the cavity of the heart, and forming the well-marked 

 heart body. Compare the blood space in this specimen with that of the 

 specimen drawn in Fig. 38. x 96. 



Fig. 40. — Blood-corpuscles of A. Grubii. Many of these were found in 

 sections of the heart shown in Fig. 38, embedded in the mass of blood con- 

 tained in the cavity of the heart. X 1000. 



Fig. 41. — A portion of the tip of one of the invaginated processes of the 

 heart-body of A. Grubii (from a specimen about 150 mm. long), to show 

 the endothelium {Endlh.), the muscle layer {ninsc), and the core of loosely 

 arranged peritoneal cells {Per. C). The last are either vacuolated or 

 granular, owing to the presence of chlorogogenous granules in the protoplasm. 

 X 470. 



Fig. 42. — A similar process from the heart-body of a full-grown specimen 

 of A. marina, showing the endothelium, the muscle-fibres, and the peritoneal 

 cells. X 240. 



Fig. 43. — Another process from the same specimen of A. marina. In this 

 and several other processes there is a large collection of chlorogogenous granules 

 in many of the cells. These are shown in the figure in red, but they are naturally 

 of a yellow colour. In many of the cells the granules are aggregated into 

 small heaps, sometimes lying in a vacuole. The cell-outlines of the cells 

 which are loaded with chlorogogenous granules are difficidt to distinguish. 

 The protoplasm of many of the other peritoneal cells is granular, but the 

 granules are very small and of a different nature from the chlorogogeu 

 granules. X 600. 



Fig. 44.— Dissection of a specimen of A. Grubii from Port St. Mary, to 

 show the general characters of the internal anatomy. The special features 

 shown are the three anterior diaphragms, the buccal musculature, the septa 

 present in the posterior portion of the animal, the alimentary canal, the 

 vascular system, and the nepliridia. The vessel which passes from the dorsal 

 vessel to the first nephridium is shown on the right side, but omitted on the 

 left side. The second nephridium derives its blood-supply from the dorsal 

 longitudinal vessel (Z). L. V.). The oblique muscles which are present in the 

 septate region of the body are very narrow bands about \ mm. in diameter, 

 and cannot be shown in the drawing. Note the two strands of connective tissue 

 [Comi. T/S.1.) attached to the alimentary canal and to the body-wall at the level 

 VOL. 43, PART 3. NKW SERIES. QQ 



