254 Part III. — Twenty-fifth Annual Report 



Fig. 46. View of outside of byssus-pit, showing the outer (o-c) and inner collars 

 (i-c and i'c). 

 ,, 47. Mussel that had a purple-coloured frill to the inhalent aperture (in), 

 and a purple- coloured exhalent siphon (ex). 



PLATE XVII. 



Fig. 48. Byssus root (r), thread (t), shreds of secretion (sh). 



,, 49. Diagram of the relation of septa (sep) and the films (f), root (r). 



,, 50. Root of a new byssus showing the beginning of the byssus-stem (b) and 

 the first threads laid on overlapping one another. The ragged edge 

 of the bases of the first threads are seen at (r). Cam. luc. Obj. 

 a and Oc. 2, Zeiss. 



„ 51. Transverse section of the foot of the mussel — gland (g), yellow gland 

 (y-g), purple gland (p-g), muscle (m), longitudinal muscle (1-m), 

 median vessel (m-v), lateral vessel (1-v). (Fig. 55 shows an element 

 of the purple gland. ) 



,, 52. Stem of byssus showing the formation of nodes by the overlapping of 

 the bases of the threads (t), stem (st), root (r). 



,, 53. Drawing of byssus thread — base of attachment to byssus-stem (b), disc 

 for attachment to foreign body (d). 



,, 54. Transverse section of the groove (gr) of the foot — cuticle (ct), columnar 

 pigmented cells (c-1), purple-coloiu-ed glands (p-g), secreting surface 

 (sec-s), secretion (sec). Cam. luc. Apoch. Obj. 8 mm., Oc. 2, Zeiss. 



,, 55. Element of purple gland of foot. Vide Fig. 51. 



,, 56. Mussel showing inhalent aperture (in) and exhalent siphon (ex) widely 

 open. May, 1907. 



,, 57 and 58. Byssus-stem (st) showing the attachment of the bases of threads (t). 

 The bases overlap and are of one size. The stem has no rind. The 

 root has apparently brought away some of the septa from the byssus- 

 pit. Cam. luc. Obj. a and Oc. 2, Zeiss. (Another view of this 

 bj'ssus-stem is shown in Fig. 58.) 



,, 59. Drawing to show the glands (g) which surround the byssus-pit, and in 

 which the muscles (m) are embedded — foot (f). 



,, 60. Longitudinal section through the byssus-pit and foot to show the connec- 

 tion of the median vessel (m-v) with the end depression of the 

 groove (gr) — purple gland (p-g), yellow gland (y-g), muscle (m), 

 septum (sep). 



,, 61 and 62. Parts of film. Cam. luc. Obj. a and Oc. 2, Zeiss. 



,, 61a. P]gg of Faroe mussel at 10.30 a.m., June 4. 



,, 63. Byssus-pit split longitudinally to show muscles (m), gland (g), byssus- 

 pit (by-p), groove (gr), byssus stem (st). 



,, 64. Diagram to show the relations of the lilms (f) and the tips of the 

 septa (sep). The tips of the septa unite with the films, but they 

 do not go far into the stem. 



,, 65. Section of byssus-pit — position of byssus (by), collar of pit (c), groove 

 (gr), film (f), septa (sep), posterior deep groove (d-gr), yellow gland 

 (j'-g), glandular tissue (g). 



,, 66. Byssus pit laid open — groove of foot (gr), byssus (by), outer collar (o-c), 

 inner collar (i-c). 



PLATE XVIII. 



Fig. 67. Track of mussel — discs without hairs (p). 

 M 68. ,, ,, 



,, 69. Mussel attached to glass by discarded byssus. 



PLATE XIX. 



Fig. 70-80. Successive drawings of the same mussel to show the formation of 

 byssus-threads — disc without a hair (p). 



,, 81. Drawing of a mussel that raised itself on end on the bottom of a box, 

 and attached itself by numerous threads in 18 hours. 



,, 82. Mussel with byssus slack. 



,,83. ,, ' ,, hauled tight. 



,, 84. Drawing of Faroe mussel showing heap of spawn at its round end- 

 spawn (sp). 



PLATE XX. 



Fig. 85. Diagrammatic section of byssus-pit to show septa and films. 

 ,, 86. Diagram to show the morphology of the septum, viz., that it is formed 

 by the united tendons of three sets of muscles 



