236 ZOOLOGY. 
Indeed most mollusks spawn in the summer. Species of 
Reilia, Galeomma, and Montacuta are viviparous. 
Some bivalves get their growth in a single year. The fresh- 
water muscles live from ten to twelve years and perhaps 
longer; while’ 7ridacna gigantea probably lives from sixty 
years to a century. Of about 14,000 known species of 
Lamellibranchs, from 8000 to 9000 are fossil. 
Cuass I.—LAMELLIBRANCHIATA., 
Bilaterally symmetrical mollusks, with two valves lined by the mantle, con~ 
wected by a dorsal hinge and ligament; no head ; mouth unarmed, with 
two pairs of labial palpi ; intestine coiled in the visceral mass, usually 
passing through the ventricle, and always ending at the posterior, usually 
siphon-bearing end, of the body. Foot small, sometimes nearly wanting; 
containing two ears (otocysis). Usually two pairs of large leaf-like gills on 
each side of the visceral mass. Sexes usually in separate individuals, 
Embryo passing through a so-called morula, gastrula, and free-swimming 
veliger condition. 
Order 1. Asiphonia.—Body-wall or mantle without siphons. Shell 
sometimes inequivalve. (Ostrea, Anomia, Pecten, Melea- 
grina, Mytilus, Arca, Trigonia, Unio, and Anodonta.) 
Order 2. Siphoniata.—Siphons present. Shell equivaive. (Chama 
Tridacna, Cardium, Venus, Mactra, Tellina, Solen, Clava- 
gella, Aspergillum.) 
Laboratory Work.—In dissecting the clam, etc., the work should be 
performed under water, in a dissecting trough. One shell should be 
removed by cutting the adductor by a pointed scalpel, the mantle dis- 
sected off and thrown aside, s0 as to expose the gills, heart, and kid- 
neys. In dissecting the nervous system it is well to introduce a probe 
into the mouth, and then cut down towards it from above, when the 
white supracesophageal ganglia or ‘‘brain” will be found, and the 
other ganglia can thence be traced by the commissures leading from the 
“brain.” To find the pedal ganglia and otocyst, cut the foot vertically 
in two. The heart can be readily found, and the large vein at the base 
of the gills, but the arterial and venous systems can only well bo 
studied after making careful injections. For ordinary or even quite 
fine injections, Sabatier used a mixture of lard and turpentine, some- 
times adding a little suet or wax to thicken the paste, which was 
colored chrome yellow, vermilion, or blue. For histological exami- 
nation he used essence of turpentine, colored as before, or gelatine 
