Gastroch^nid^. MOLLUSCA. Saxicava, 
368 
of use. Thej' break down and destroy fragments of 
wrecks and floating timber which might otherwise act 
as dangerous impediments to navigation ; for wood when 
entirely submerged, i.s almost indestructible under the 
mere influence of water. The celebrated engineer, 
Mr. Brunei, told Sir David Brewster that his plan 'of 
tunnelling the Thames was suggested to him by wit- 
nessing the boring of a Teredo. The Ship-worms are 
ovoviviparous animals, and are said to be of separate 
sexes. At certain seasons it has been observed that the 
female carries her eggs in the folds of her respiratory 
organs, and that they are ejected from the siphonal 
tube, after the milt of the male, floating in the water, 
has fecundated them.* The young swim freely about, 
and are even able to float on the surface of the water. 
Family— GASTROCH.ai:NIDJS. 
This family is composed of species which are bor- 
rowers in mud or stone. The shells are thin, gaping 
in front, equivalve. The hinge is destitute of teeth, 
and the valves are united by a narrow ligament ; they 
have no accessory plates on the dorsal margin. The 
animals live inclosed in calcareous tubes, or imbedded 
in mud or stone. The valves of the shell are either 
free or cemented to the shelly tube. The tube Itself 
appears to be foi-med by the animal to protect its elon- 
gated and partly naked body from the roughness of the 
sand, or the rock, in the holes of which they reside. 
Genus Aspergillum (= Penicillus ). — This genus, 
the “ Watering-pot,” as it has been called, has the two 
valves of its shell, which are oval and equivalve, im- 
bedded in the lower part of the tube, the beaks alone 
being visible externally. They are so small, and so 
completely imbedded in the substance of the tube, that 
they are generally overlooked, and the tube itself is the 
part looked upon as the real shell. This shelly tube 
is elongated, cylindrical, gradually becoming narrower 
posteriorly, or at the siphonal end, at which extremity 
it is open and sometimes ornamented with shelly expan- 
sions, like ruffles. At the anterior end it is club- 
shaped, and closed by a convex disc pierced with 
numerous tubular holes (like the rose of a watering- 
pot — hence the English name), with a fissure in the 
centre, and a circular frill of tubes round the periphery. 
The species, which are not numerous, seven or eight 
only having been described, are found imbedded in 
sand and mud at low water, on the shores of tropical 
countries; the closed disc with the tubular holes being 
below the surface, and the simple or ruffled siphonal 
extremity projecting about an inch above it. The 
genus is represented in Plate 10, fig. 10, by A. Deles- 
sertianum. 
Genus Clavagella (= Bnjopa ). — This genus has 
only one of the valves (the left one) of the shell im- 
bedded in the shelly tube, the other being free. They 
are oblong, flat, often irregular, and inequivalve. The 
tube is cylindrical, varying in length, but generally 
much shorter than that of Aspergillum. It is open 
* Note. — M. Quatrefages finds that a weak solution of 
mercury thrown into the water destroys this milt, and con- 
sequently prevents the fecundation of the eggs, nipping the 
young mollusks in the hud. — Lewes. 
posteriorly, the margin of the opening being sometimes 
furnished with a succession of fringes, and is closed 
below or at the anterior end by a disc with a minute 
central fissure, and bordered with branching tubuli. 
Genus Fistulana {—Chcena ). — This genus has 
both the valves of the shell free. They are elongated, 
irregular, inequivalve, and Widely gaping. They are 
contained within a shelly tube, which is round, taper- 
ing, and open at the anterior extremity, straight, trans- 
versely striated, closed at the lower end when complete, 
and thickened or club-shaped. It is furnished with a 
perforated diaphragm behind the valves. 
Genus Gastrocha;na. — This genus has both the 
valves of the shell free also. The valves are wedge- 
shaped, regular, widely gaping in front, and close behind. 
The species perforate shells and limestone, and line the 
crypts which they make with a layer of shelly matter. 
The holes made by one of the common species, Gas- 
trochcena modiolina, are about two inches deep and half 
an inch in diameter. The external orifice is flask- 
shaped, and lined with a thin layer of shell, which 
projects slightly. 
Family— SAXICAVID.^. 
The Saxicavas, as their name imports, form another 
family of Borers. They have great affinities with the 
preceding family, Gastrochcenidce ; but, though often 
perforating stones, they never form shelly tubes or pro- 
tecting cases. They are borers, says Professor Forbes, 
“ although the habit of boring does not seem necessary 
to their existence, since we find them very commonly 
free. If there be a crevice, however, in a rock, shell, 
coral, or sea-weed, into which they can thrust them- 
selves, the}' do so ; and if near a limestone rock, per- 
forate it, and form crypts in which they live.” 
Genus Saxicava.— This genus has the shell equi- 
valve, solid, wedge-shaped, and gaping at each end — 
when young, with two minute teeth in each valve ; but 
when adult, becoming toothless. The cartilage is ex- 
ternal, thick, and prominent, and the pallial line sinuated, 
not continuous. The animals secrete a byssus. The 
species of Saxicavoe are few in number, but are very 
widely diffused, the same species even being found in 
many and distant parts of the globe. They are usually 
found burrowing in limestone rocks, though they have 
also been observed in sandstone, the animal being fixed 
by the byssus to the side of the hole it excavates. The 
question how these animals bore, has been as much 
discussed, as the Pholades already mentioned. The 
acid theory has had many supporters ; but perhaps, on 
the whole, the views of Mr. Hancock may be con- 
sidered as mostlikely to prove correct. The thickened 
portion of the mantle of the Saxicavais armed, he says, 
with a rough layer of numerous crystalline particles of 
various shapes and sizes, which are imbedded in the 
surface, and consist probably of silex or flint, either 
pure or in combination with some animal matter. 
This forms in fact a kind of file, superior, however, 
to any of our workmen’s files in this, that the surface 
keeps itself always in a proper state of roughness for 
trituration, the siliceous crystals being constantly shed 
and as constantly renewed. This thickened portion of the 
