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 {Cytherea), selected for the demonstration of the 

 general organisation of the Lamellibranchs, there are intermediate 

 modifications. The common mussel is the type of a family (Mi/tilacea) 

 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 C/iamacea the margins of the paUial 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 Tellinidce {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 

 Inclusa ; 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 

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

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

 the razo't-shell ( 5'o/e7i siliqua) ; they are longer and partly separate in 

 the shorter-bodied solens. 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 proporflon of that vermiform mollusk, is unprotected by 

 the bivalve-shell. A well-marked modification of the mantle is pre- 

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

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

 of the sipiion, 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 boring 

 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, arranged 



L L 



