STRUCTURE AND PHYSIOLOGY OF THE MOLLUSCA. 19 
inversion of a glove-finger; the branching gills of some of 
the sea-slugs, and the tentacles of the cuttle-fishes are also 
eminently contractile.* 
The inner tunic of the ascidians (Fig. 8, t) presents a beautiful 
example of muscular tissue, the crossing fibres haying much 
the appearance of basket-work; in the transparent salpians, 
these fibres are grouped in flat bands, and arranged in charac- 
teristic patterns. In this class (tunicata) they act only as 
sphincters (or circular muscles), and by their sudden contraction 
expel the water from the branchial cayity. The muscular foot 
of the bivalves is extremely flexible, having layers of circular 
fibres for its protrusion (Fig. 18, /), and longitudinal bands for 
its retraction (Fig. 30 *); its structure and mobility has been 
compared to that of the human tongue. 
In the burrowing shell-fish (such as 
solen), it is very large and powerful, and 
in the boring species, its surface is 
studded with siliceous particles (spicula), 
which renders it a very efficient instru- 
ment for the enlargement of their cells. 
(Hancock); In the attached bivalves it 
is not developed, or exists only inarudi- = Fig. 13. Dreissena.t 
mentary state, and is subsidiary to a gland which secretes the 
material of those threads with which the mussel and pinna 
attach themselves (Fig. 13). These threads are termed the 
byssus ; the plug of the anomia and the pedicel of terebratula 
are modifications of the byssus. 
In the cuttle-fishes alone we find muscles attached to internal 
cartilages which represent the bones of vertebrate animals; the 
muscles of the arms are inserted in a cranial cartilage, and those 
of the fins in the lateral cartilages. 
Muscles of a third kind are attached to the shell. The valves 
of the oyster (and other mono-myaries) are connected by a 
single muscle; those of the cytherea (and other di-myaries), by 
two; the contraction of which brings the yalves together. 
They are hence named adductors; and the part of the shell 
* The muscular fibres of molluscs frequently present the transverse stripes which 
characterise vocuntary muscles in the higher animals. Striped muscular fibre has been 
observed in Salpa (Huxley); and in Waidheimia australis by Hancock; a strict search 
wus made by that able anatomist for the purpose of discovering such fibre amongst the 
hingeless brachiopods, but without success. - Striped fibres have been scen in the 
gasteropods. 
T Fig. 13, Dreissena polymorpha (Pallas sp.), from the Surrey timber-docks 
f, foot ; b, byssus, 
