ANATOMY OF THE PEARL OYSTER 49 



disc-pit. During the withdrawal of the foot the edges of the byssal groove open to 

 permit of the uewly-fbrmed byssal fibre passing out. Under favourable conditions 

 this operation is performed again and again, until at last from 50 to 70 fibres are 

 formed, constituting a wonderfully strong cable of attachment. In 3-year-old 

 oysters the byssus is so strong that the twist and wrench necessary to break it off 

 requires a distinct effort. The divers dislike fishing pearl oysters which are attached 

 individually to rocky surfaces, as the effort of wrenching them off reduces the result 

 of the day's work both by taking longer time and also because of the weariness 

 induced by the hardness of the work. After a few days' fishing on a rocky bank the 

 divers' hands become painfully lacerated unless they take the precaution, as many do 

 now, of using roughly made gloves. From the age of 4 to 5 years the strength of 

 the byssus decreases, rendering the older oysters more easy of detachment. 



Structure of the Byssus. On examining a sloughed byssus we find that the 

 individual fibres arise from an ovate, laterally compressed "root" (Plate III., fig. 23 ; 

 sometimes it is forked with laterally spreading groups of fibres), the surface whereof 

 is corrugated or laminated in correspondence with the parallel folding or grooving, 

 which is characteristic of the inner surface of the walls of the byssal gland 

 (Plate VIII. , fig. 5). Tli is mass can be resolved into branched or pennate fibres, the 

 twigs of which penetrate between the lamella? of the gland and come into close 

 relation with the surrounding muscle bundles. Each byssus fibre, outside the body, 

 is distinctly flattened laterally, and can be readily frayed into a number of longitudinal 

 fibrils. Each terminates distally in an oval disc by which attachment is made to 

 rocks, old oysters, and other suitable bodies. The fibres are of a lustrous deep bronze- 

 green colour, growing paler as they enter the root. When first formed, however, 

 they are pale yellow, gradually becoming of the characteristic green tint in the course 

 of the ensuing 48 hours. 



The byssus is markedly elastic and very tough, and the disc is so firmly attached 

 that if sufficient tension is applied either the substance to which the disc adheres 

 gives way or the strands themselves break. It is a most rare occurrence for the 

 "root" to be torn out a fact that is of the greatest importance in the cidtivation 

 by transplantation of the pearl oyster, as otherwise the pearl oysters dredged for 

 transplantation would be so injured during the operation that death would ensue in 

 the majority of cases. As it is, the wrenching off of pearl oysters is found in practice 

 to induce no ill effects. An hour afterwards, under favourable and natural conditions, 

 the pearl oyster begins to slough the root of the ruptured byssus and may, indeed, 

 actually make re-attachment by means of a new byssus within this period. 



The approximated ends of the two retractor muscles are attached to the hinder end 

 of the byssal gland (Plate III., fig. 24, Ret.). The structure of the byssus gland, as 

 seen in section, is shown in fig. 5 on Plate VIII. It is divided into two halves, placed 

 right and left, and each formed of parallel layers of glands opening into narrow folds, 

 in which the secretion forms long, and in places convoluted, threads. The byssus 



H 



