LAMELLIBRANCHIATA. 513 



of the skin, especially about the inhalent and exhalent orifices, are 

 very irritable. 



Between the freely open state of the mantle in the oyster and 

 similar monomyary bivalves (Ostracea), and its condition in the 

 dimyary bivalve {Ci/therea), selected for the demonstration of the 

 general organisation of the Lamellibranchs, there are intermediate 

 modifications. The common mussel is the type of a family (3Ii/tifacea) 

 in which the mantle is widely open anteriorly, the margins of the 

 lobes being united together posteriorly, except for a small space 

 forming an inlet for the respiratory currents and an outlet for the 

 excrements. In the Chamacea the margins of the pallial lobes 

 coalesce, leaving a small anterior aperture for the foot, a second 

 smaller one for inhaling the respiratory and nutrient currents, and a 

 third posterior orifice for excretion. The families typified by the 

 Venus and Mactra have the two latter orifices produced into a 

 siphonic tube, and the anterior or pedial aperture corresponds in 

 width with the superior size of the foot. In the TelUnidce {fig. 187.), 

 the siphons are separate and can be much elongated. The modifi- 

 cations of the mantle are essentially the same in the family called 

 Incliisa ; but the narrower and longer figure of the body occasions a 

 greater proportion of the confluent margins of the mantle between 

 the anterior pedial and the posterior siphonic apertures, whereby the 

 mollusk, especially when the foot is small, becomes inclosed in a 

 membranous tube or sheath. The siphons are short and united in 

 the razor-shell (Solen siliqua) ; they are longer and partly separate in 

 the shorter-bodied sohens. In the PanopcEa the long common siphonic 

 tube is covered by a thick rough epiderm and is not wholly retractile. 

 The still longer siphon of the shipborer {Teredo), which forms a 

 very great proportion of that vermiform mollusk, is unprotected by 

 the bivalve-shelL A well-marked modification of the mantle is pre- 

 sented by the Pkoladoinya, which has, besides the pedial and two 

 siphonic apertures, a fourth orifice at the under part of the base 

 of the siphon, leading by a valvular protuberance into the anterior 

 of the pallial cavity. This additional aperture co-exists with a 

 second small muscular process or foot, which is bifurcate at the 

 extremity. 



The bivalve shell of the Lamellibranchs offers, as might be ex- 

 pected, many modifications corresponding in general with those of 

 the mantle ; but otherwise related, in a few species, with boriuijc 

 habits and a peculiar locality, other calcareous parts in a tubular or 

 other form being then usually superadded. The shell consists es- 

 sentially of an organised extravascular combination of gelatinous 

 membrane and calcareous earth, chiefly carbonate of lime, arran"-ed 



L L 



