INVERTEBRATE ZOOLOGY. 131 



about a center, the apex. Is the spiral wound towards the right (with the 

 hands of a watch) or in the opposite direction ? Are all specimens alike in 

 this ? The separate sj)irals are divided from each other by a crease, the suhire. 

 The open end of the last spiral is termed the " Month'' and its free edge, the 

 " lip." Notice the slightly curved lines upon the spirals, at right angles to 

 the suture. What is their significance ? Do they correspond to anything on 

 the surface of a clam shell? On the head notice :— The long tentacles {omma- 

 tophores) which bear at their tips the eyes and probably the olfactory organs ; 

 the short tentacles : the genital opening on the right side, at the base of the 

 ommatophore (behind the short tentacle in H. albolabn's) : the mouth, sur- 

 roiTnded by soft lobes, the lips. Probe the mouth gently with a needle and 

 feel a hard chitinous dorsal piece, the jaiv or maxilla. Beneath the margin of 

 the shell on the right side, notice an opening, the breathing jyore or pneumo- 

 .sfo?Jie, which opens into a large respiratory sac, the so-called "lung." The 

 intestine runs along tlie wall of this sac and opens at the edge of the pneumo- 

 stome. The nephridial opening is at the same place, but cannot be demon- 

 strated from the exterior. During life the pneumostome is under the control 

 of muscles. The foot is provided with a flat creeping sole, posteriorly pro- 

 longed, and running anteriorly beneath the head, nearly as far as the mouth. 



I 04. Dissection. Carefully remove the shell, beginning at the edge and 

 taking it away in little pieces. Notice that the shell has a central support, 

 the colnmella. about which the visceral sac is wound. Commencing l>elow, 

 uncoil the visceral sac, removing every vestige of columella and shell. The 

 entire visceral sac is covered by a delicate membrane, the mantle, which en- 

 closes the organs, and has a thickened edge below, the mantle ridge, which 

 marks the boundary of the shell. Pass a probe into the pneumostome. It en- 

 ters the respiratory chamber, and may be seen through its transparent wall. 

 Open this chamber with the scissors, and notice upon the right side ; — the 

 heart, enclosed in a delicate sac, the pericardinm ; the nephridial organ with 

 its duct, and the rectum, terminating in the anus. A rich net-work of blood- 

 vessels is spread over the walls. The floor of the lung-chamber is formed by 

 the body-wall, which covers the visceral organs and thus shuts off the res- 

 piratory chamber from the coelom. (cf. the branchial chamliers of the cray 

 fish and the clam.) For farther dissection make a median dorsal incision, 

 beginning just above the mouth. Carry it posteriorly through the mantle- 

 ridge and the floor of the respiratory chamber. As it is difficult to continue 

 a mid-dorsal incision upon the coiled visceral sac, the entire mantle may be 

 removed from this, thus liberating the organs. All the cavities of the body 



