368 



GastkocHjEnid^. MOLLUSCA.- 



-Saxicava. 



of use. Tliey break down and destroy fragments of 

 wrecks and floating timber which might otherwise act 

 as dangerous impediments to navigation ; for wood when 

 entirely submerged, is almost indestructible under the 

 mere influence of water. Tlie 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^NID^. 



Tliis family is composed of species which are bur- 

 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 cither 

 free or cemented to the shelly tube. The tube itself 

 appears to be formed by the animal to protect its elon- 

 gated and partly naked body from tlie rouglmess of the 

 sand, or the rock, in the holes of which they reside. 



Genus Aspergillum (= Penidlhis). — 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 

 completel}' 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 

 posteriori}', or at the siphonal end, at which extremity 

 it is open and sometimes ornamented with shelly expan- 

 sions, like ruflles. At the anterior end it is club- 

 shaped, and closed bj' 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. 

 Tlie species, which are not numerous, seven or eiglit 

 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 pirojecting about an inch above it. The 

 genus is represented in Plate 10, fig. 10, by A. Deles- 

 sertlannm. 



Genus Ci.avagella (= 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 iuequivalve. Tlie 

 tube is cylindrical, varying in lengtli, but generally 

 much shorter than that of Aspergillum. It is open 



* Note. — I\r. Qiiatrefages finds that a weak solution of 

 mercury thrown into the water destroys this milt, and con- 

 8'i-|ucutty prevents the fecundation of the eggs, nipping the 

 voung mo\lusks in the bud. — 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 tubiili. 



Gi;nus 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 extremitj', 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 Gastrocii«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- 

 trochana 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 tlieir name imports, form another 

 family of Borers. Thej' have great affinities with the 

 preceding family, GastroclKViiida: ; but, though often 

 perforating stones, they never form shelly tubes or pro- 

 tecting ca.ses. 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, they 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 Saxicava; are few in number, but are very 

 widely dilTused, 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 most likely to prove correct. The thickened 

 portion of the mantle of the Saxicai'a is armed, he says, 

 with a rough laj'er 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 anj' of our workmen's files in this, tl at the surface 

 keeps itself always in a proper state of roughness for 

 trituration, the siliceous crystals being constantly shed 

 andasconstantly renewed. Thisthiekenedportioa of the 



