INVERTEBRATE ZOOLOGY. 137 



6. Miiscidar fibres. 



A few fibres may be isolated by picking them to pieces on a slide in 

 a drop of water. They may then be temporarily monntetl to show 

 the striations. Stained musciilar fibres will bring out the nuclei of 

 the investing membrane {Sarcolemma). 



Type XIX.— My a Ed U I Is (or Venns Mercatoria). 



99. Obtaining ami preparing. Clams may be obtained inland at the fish- 

 markets and restaurants except during the coldest weather. The forms 

 named above are distinguished by fishermen as "soft" and •'hard-shelled 

 clams " respectively. The first, Mya. is the one described here, but Venus 

 will be found to correspond very well. If the clams are laid in warm fresh 

 water for some time (the water is best left in the si;n) they wall become passive 

 or dead with the parts expanded. For histological details, as with other 

 forms of large size, the separate organs should be removed from a fresh spec- 

 imen and preserved with special reference to the object in view. 



I OO. General disseciion. Is the animal bilateral ? Can you find the bi- 

 lateral plane ? Are the shells dorsal and ventral, or right and left? Is the 

 animal compressed or depressed? Each shell has a knob at the top, the umbo, 

 from which proceed a series of concentric curves, the lines of groivth, each 

 representing the edge of the shell at some former time. Were the shells 

 always of the same shajie ? The anterior end of the shells is more rounded, 

 the posterior more prohjnged. Back of the umbo is an elastic ligament, the 

 hinge ligament, which tends to open the shells. Against this work two inter- 

 nal muscles, the addnetors. which stretch across from shell to shell. When 

 these are relaxed or cut, the shells open through the force exerted by the 

 hinge ligament. Each shell is lined by a delicate membrane, the pallium or 

 mantle, the edges of which project a little beyond the edges of the shell, and 

 are grown together, except at the anterior end, where a little space is left 

 through which projects a small muscular locomotive organ, the foot. Poste- 

 riorly the mantle forms a double tube, the siphon, which is capable of ex- 

 tending twice the length of the shell, or may be wholly retracted. Look for 

 two openings at the end. The ventral one, towards the free edges of the 

 shell, is the incurrent opening, branchial sijihon, and the dorsal one, towards 

 the hinge, the excurrent, cloaccd siphon. Place the animal upon its left side 

 (right side ujipermost) and slip a scalpel along the edge close to the right 

 shell, separating it from mantle and siphon. Then reaching in further, and 



