aperture when the operculum is in place. We learned from Professor 

 Parker that the operculum varies greatly in different snails, some being 

 horny, while others are calcareous like the shell. They are nearly all 

 formed on the plan of a spiral, although in some snails, as in the oper- 

 culum of Nerita, it is not at first apparent. If the operculum of the 

 drill be closely examined, it will be seen to have grown from a small 

 nucleus by the addition of new matter, as did the shell of the animal. 



We now placed the drill into the pool 

 again, to see if it would not come out of 

 its shell and crawl about. The first thing 

 to appear was the brownish operculum, 

 which was pushed out of the aperture, 

 and then a flat, muscular disk appeared, 

 which was the foot, or organ of locomo- 

 tion. The head next appeared, with its 

 pair of tentacles, which bear the black 

 eyes near their bases. Soon the animal 

 was fully extended, and crawled about on 

 its muscular foot. 



" If we could take the time to dissect 

 the drill," said the Professor, "we would 

 find that it is a perfect animal, having a 

 brain and nervous system ; a digestive 

 system made up of a mouth, teeth, sali- 

 vary gland, crop, stomach, and intestine ; 

 a heart with veins and arteries carrying 

 the blood to all parts of the body ; a com- 

 plicated set of muscles for all its organs, 

 and a gill for the purpose of respiration. 

 In fact, this lowly mollusk is as perfectly 

 fitted for its work in life as are we for 

 ours, and its organs are as perfect, though not, of course, as highly devel- 

 oped. Like nearly all marine snails, it breathes by means of a gill, or 

 branchium, made up of a long central stem or rib, from which a number 

 of leaf-like filaments extend, like the teeth of a comb. The fresh-water 

 breathers also have the same kind of a gill, as we have already learned." 

 As the Professor was thus talking, he suddenly stooped over and 

 picked up a quahog shell with a peculiar little round hole in it. "Aha ! ' 

 said he, "here is some of the work of the drill, the animal of which is 

 carnivorous, and lives by boring holes in bivalve shells, through which 

 it may suck the juices of the clam. The drill applies its mouth to the 



86 



General anatomy of periwinkle, 

 Littorina littorea; the animal removed 

 from its shell, and the branchial cavity 

 and back laid open, a, anus; b, bran- 

 chiuni, or gill; c, heart: e, stomach; f, 

 liver: g, nervous ganglia surrounding 

 the oesophagus; h, iDiliary canal; i, 

 intestine; k, buccal mass; 1, lingual 

 coil; m, columella muscle; n, aorta; 

 ce, oesophagus; r, rostrum, or proboscis; 

 s, salivary gland; t, foot; x, kidney, or 

 renal organ; y, mucous gland; z, eye. 

 (Woodward.) 



