256 ZOOLOGY. 



Indeed most mollusks spawn in the summer. Species of 

 Kellia, 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 Tridacna gigantea probably lives from sixty 

 years to a century. Of about 14,000 known species of 

 Lamellibranchs, from 8000 to 9000 are fossil. 



Class I.— LAMELLIBRANCHIATA. 



Bilaterally symmetrical mollusks, with two valves lined hy the mantle, con- 

 nected hy a dorsal hinge and ligament ; no head ; mouth unarmed, with 

 iivo pairs of labial palpi ; intestine coiled in the visceral mass, usually 

 passing through the ventricle, and always ending at the posterior, usually 

 siphon-hearing, end of the body. Foot small, sometimes nearly wanting, 

 never used as a creeping disk. 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, Anemia, Pecten, Melea- 

 grina, Mytilus, Area, Tiigonia, Unio, and Anodonta.) 



Order 3. Siphoniata. — Siphons present. Shell equivalve. (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, so 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 be 

 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 



