HABITS AND STRUCTURE OF ARENICOLA MARINA. 39 



Vac. Vacuole. Vac. C. Vacuolated cells of the heart. V. Nph. Ventral lip 

 of uephrostome. V. V. Ventral vessel. I.II.III.IF.&.C. Somites beginning 

 with the first chsetigerous. 



PLATE 1. 



Fig. 1. — The anterior end of a large specimen of the " Laminarian " 

 variety seen from the left side, to show the external features, the seg- 

 mentation of the body-wall in relation to the internal metamerism, the 

 nephridial apertures, and the commencement of the branchial region. The 

 achsetous region following the fully everted buccal mass {Biicc. M.) extends 

 forwards as far as the groove indicating the insertion of the first diaphragm 

 dorsally (Dp/im.^). We have considered the first chsetigerous annulus and the 

 annulus behind this, as composing the first chsetigerous somite {I), although 

 we are fully aware that, owing to the obliquity of the first diaphragm, and the 

 absence of landmarks in the achsetous region in front of this septum, it is 

 somewhat hazardous to delimit this first cheetigerous somite. X |. 



Fig. 2. — View from the right side of two somites from the anterior part of 

 the branchial region of a specimen of the " Laminarian " variety 7 inches long. 

 The fourth gill is shown in detail, while the third and fifth are cut down to 

 the base of the main branches. The large size of the spreading branches 

 and the somewhat pinnate arrangement of the lateral twigs distinguish the 

 gill of this variety of A. marina from that of the ordinary shore lugworm 

 seen in figs. 3 and 4. The webbing at the bases of the branches is generally 

 much more marked in old black examples than in immature dark red 

 specimens such as the present. X 14. 



Fig. 3. — Fifth gill of the right side of a shore lugworm 8 inches long, to 

 show the features characteristic of the littoral variety of Arenicola 

 marina. The branches are united by extensive connecting membranes, 

 between which the blood-vessels of the gill are faintly visible. X 14. 



Fig. 4.— The first gill of the right side from the same specimen as Fig. 3. 

 The ventral branches -are apparently the last to develop, and are only just 

 budding off the secondary leaflets. X 14. 



PLATE 2. 



Fig. 5. — Dissection of a large "Laminarian" variety, to show the general 

 characters of the internal anatomy (conf. pp. 9 to 10). The body-wall has 

 been cut along the mid-dorsal line, the flaps pinned back, and the alimentary 

 canal turned over to the left side. The special features shown are the 

 vascular system, the nephridia, the septa, and muscles, x 2. 



