BYSSAL SECRETl()?rS. 
321 
Fig. 613. — Transverse section of a furrow 
of the byssogenous cavity of Modiolaria 
discors^ X 250 (after Cattie), showing the 
byssal filament and the formative secretion 
passing from the byssogenous cells between 
the epithelial cells to the byssal furrow, 
h. byssal filament ; h.c. byssogenous cells, 
their secretion flowing between the epithelial 
cellules, e.z. 
secretions permeate between the epitlielial cellules and accuinnlate 
within the byssal cavity, flowing therefrom by a duct into a semilunar 
groove or furrow invested by large 
mncus cells which runs along the 
ventral edge of the foot and is 
capable of being converted into a 
closed canal, wherein the chitinous 
secretion is moulded into threads 
or filaments, which harden quickly 
on exposure, their adherent puncti- 
form distal extremities being 
formed within a cup-like depression 
at the anterior end of the byssal 
furrow, within which is secreted 
the adhesive substance by the aid 
of which attachment is made. 
The byssus is composed of a bundle of tough chitinous fibres, 
resembling horn in some of its physical characteristics, but adhesive 
when first formed and therefore available as a means of anchorage 
to foreign objects. It can, however, if necessary be thrown bodily off 
and replaced by a new one when the animal desires to change its 
position, or the mollusk can 
ascend any smooth vertical sur- 
face by alternately fixing and 
rejecting a series of successively 
secreted byssal attachments, 
the rejected byssal tufts of 
Dreissensia being regarded by 
Dr. Mbrch as constituting the 
Tiihnlaria cas^pia of Pallas. 
This gland when highly developed, as in Dreiasensia, exercises a 
prejudicial influence upon the development of the foot itself, which 
may cease to be functional as an efficient locomotory organ, and 
diminish in size in inverse ratio to the development of the byssus, for 
which it comes eventually to serve merely as a support, the posterior 
pedal retractor muscles even becoming fixed to the gland and 
functioning as byssal I’etractors. 
In the UnUmklce, Sphwriidw, etc., the organ is more or less vestigial 
in the adult, though present and functional in the young. In 
Fig. 614. — J\Iytihis cduh's with e.xtended foot 
in process of attaching a byssal filament (after 
Meyer and Mdbius), 
13/1/1900. 
V 
