of the Fishery Board for Scotland. 245 



cells of the epithelial walls, partly by the glandular cells which lie in 

 the surrounding tissue, their ducts passing between the epithelial cells. 

 The secretion takes the form of the cavity, and is thus held fast as with 

 roots by the numerous lamellse which occupy the shelves. As the amount 

 01 the secretion in the cavity increases theso lamellae are pressed into the 

 duct, where they unite to form the main stem of the byssus. When a 

 bivalve attaches itself it forms a bysses thread in the groove [of the footj 

 which fuses with the end of the main stem." 



TuUherg says that the groove of the foot, the side of the byssus-pit, and 

 the septa are clothed with cilia. These I have not been able to see. 

 Tullberg throws doubt on Sabatier's observation that the middle vessel 

 of the foot opened to the exterior in the depression at the end of the 

 groove. I have been able to confirm this. Tullberg describes at length 

 and discusses the function of the main gland-masses present in the foot. 

 The main ghuid is white (yellow from outside the foot) ; above it there 

 is the green (or puiple) gland, and on either side of the groove a less 

 gland somewhat similar to the latter. He says that the walls of the 

 byssus pit as well as all their sheaths are principally formed of 

 connective tissue, to which the muscles which surround the pit are 

 attached. In the cmmective tissue, and between adjacent parts of the 

 muscle-bundles, are found glands similar in nature to the white gland of 

 thft foot, although they are separated a little more from one another. 

 The root of the byssus consists of a multitude of thin structareless 

 lanif'llse, one in each compartment. They can be easily split into threads. 

 They are folded in many plaits, where the byssus pit narrows to form a 

 neck. He divides the lamelhe into primary and secondary lamellae. The 

 septa in the hind part of the pit end sooner than in the other parts, and 

 the lamellae, according to Tullberg, are enveloped by the lamellae from the 

 front part of the pit, forming an outer binding rind to the byssus 

 stem. In section this is seen to be composed of concentric rings. Each 

 of the byssus threads is attached to one of the above-mentioned con- 

 centric layers, and appears to be formed simultaneously. I do not agree 

 with this, as is shown below. No part of the lamellae takes part in the 

 formation of the tliread. TulU'erg says that the thread appears to 

 consist of a quite homogeneous substance, although in sections the surface 

 stains differently with carmine than does the internal part. But this 

 njiiy be due to the outer surface having been acted on by sea water, not 

 to any difference in origin and quality. The thread splits up into fine 

 threads, just as do the lamellae and the stem. Miiller had described 

 seven openings from the foot-gland into the end depression. Tullberg 

 found one only. I have observed a number of slits in the floor of 

 the depression, but I did not make out their connection to any gland. 

 The lamellae grow through the flow of secretion. He says that it is 

 possible that the secretion of the green gland may be used to coat the 

 thread with a fine skin. 



I have examined the structure of the byssus and the foot. My 

 results are in general agreement with those of Tullberg, except in one 

 or two points. For example, I do not agree that there is any 

 enveloping rind to the stem such as he describes. The base of each 

 thread forms such an envelope, and, where the threads have been made 

 one after the other in rapid succession, a thick rind is formed by 

 their overlapping bases. On those parts of the stem where there are no 

 hairs attached there is no rind. 



I will discuss the structure of the foot first, then the byssus-pit and 

 finally the thread and its formation. 



Foot (figs. 3. 43, 45, 60). — The foot is a brown or pur[>le-coloured 

 organ. It is very muscular, capable of retraction into a small body, 



