Miscellaneous. 319 



cavity are occupied by gland-cells whicli discharge their secretion 

 into the groove, i. e. the cavity, and furnish the material for t^e 

 formation of the byssus. Ko such gland-cells are present, however, 

 as I shall demonstrate more in detail in my completed memoir. / 



The groove vphich traverses the foot shows two distinct parts, an 

 outer one of simply iissure-like form, and an inner one with a 

 crescentic transverse section. This is quite in open connexion with 

 the fissure, and is to he regarded merely as the sudden dilatation of 

 the fissure towards the two sides. By the approximation of the 

 margins of the fissure it can be closed so as to form a complete 

 canal, which is called the crescentic canal from the form of its 

 transverse section. It is exclusivelj^ in this part of the groove that 

 the byssal threads originate as a cuticular formation of the epithelium 

 which lines the canal. This is not a vibratile epithelium like that 

 which forms the surface of the fissure, the processes which are seated 

 upon the epithelial cells of the canal are the byssal substance formed 

 by them, but not vibratile cilia, for which they have hitherto been 

 taken. 



Two characters accentuate the distinction between the epithelium 

 of the canal and the vibratile epithelium of the fissure. In the 

 latter the cilia are seated upon a cell-membrane, which in transverse 

 section is distinctly recognizable by a double contour. In the former, 

 however, only a simple line appears beneath the processes, and this 

 forms the boundary between the byssal substance and the epithelial 

 cell. Further, each of these epithelial cells in the canal has only 

 one process, while in the vibratile epithelium a number of cilia are 

 seated upon each cell. 



As already indicated a byssus consists of a stem with its roots, 

 and byssal threads seated upon the stem. 



According to the secretion-theory, threads are produced only when 

 the stem is partially or completely developed, and they are attached 

 or stuck to it. Further, a different mode of production from the 

 threads is frequently ascribed to the stem, inasmuch as it is said to 

 be formed by gland- cells which differ from those which are contained 

 in the foot. This notion is, however, contradicted by observation. 

 The stem and threads of the byssus originate in the same manner, 

 simultaneously, and in immediate connexion with each other. This 

 indeed is quite natural, for the crescentic canal opens into the byssal 

 cavity, passing into it gradually, so that its wall passes into that of 

 the cavity. Now if a cuticular formation occurs it will extend over 

 the whole surface of the cavity and the groove, and in consequence 

 the threads originating in the canal will be united with the formation 

 in the cavity. 



The casting-off of the byssus is connected with a retrogression of 

 the byssal cavity. This, in its normal state, is divided at the bottom 

 by a great many perpendicular septa, standing in the longitudinal 

 direction of the animal, into so many chambers or secondary canties. 

 At the casting of the byssus these septa are reduced. From the 

 previously complicated byssal cavity is produced a simple cavity, 

 showing only a few folds in its walls. The septa originate afresh 

 only with the new formation of the byssus ; their epithelium gives 



