322 
SHELL SECRETION. 
Anodonta, the lal■^’al bj’ssal organ is a prominent and transparent tube, 
arisinir from the left side of the visceral mass and coiled aronnd the 
adductor muscle, but opening on the 
median line of the body and emitting 
one or two long filaments, which, besides 
serving to anchor the embryos within 
the brood pouch, also tend to keep the 
glochidia together by their entanglement. 
The byssal threads of Pinna, an exotic 
marine genus, are so long and delicate, 
and possess such a beautiful silky lustre 
that, according to Ogilvie, they are 
woven by the Italians into a valuable fabric, a manufacture known 
also to the ancients ; Modiola utilizes its byssal filaments to form a 
nest or retreat, strengthening the walls by attaching extraneous 
matters thereto ; while the shelly plug of Anomia, by which it is fixed 
to the ground, must also he regarded as a modified byssal attachment. 
Some exotic marine species, as Trigonia, which, when adult, 
possess neither byssus nor byssal gland, have retained the byssal 
cavity, the duct and even the retractors ; the abortion of the byssus 
is generally correlated with an increase in the size of the foot and 
with increased activity in the animal. 
The CoNCiiAL or Shell Gland originates during the Trochosphere 
stage of larval life, as an invagination of the thickened centro-dorsal 
area of the body of the embryo ; 
it secretes a viscid chitinous 
sulistance which hardens on 
exposure and is assumed to 
vestigially represent a shell 
formerly possessed by the mol- 
lusca, but which has become lost 
and is replaced by the secondary 
shell they now possess. We 
may thus speak of a Primary, 
a Secondary and even, in some 
cases, a Tertiary shell in the 
mollusca, recognizing that each 
fleeting embryonal phase is necessarily incomplete and often discon- 
tinuous owing to a su])pression in the embryo of their full development. 
Fig. G16. — Embryo of LijiiniPa stag;naii's {L.,), 
approaching the Veliger stage, highly magnified, 
to illustrate the position and a.spect of the conchal 
or shell gland (after Lankester). 
shell gland ; m. mouth ; r. rectum ; 
developing digestive gland ; /«. pronephros 
or larval kidney ; /. foot ; 7'. velum ; e.gi. cerebral 
ganglion. 
Fig. 015.— Larva or Glochidium 
of Anodonta cys^nea (L.), greatly 
enlarged (modified after Balfour). 
sh. shell, showing porous structure ; 
ad. adductor; hys. byssus; s.s. 
sensory sct.-c ; ;//. retractor muscle of 
apex of shell. 
