GASTEROPODS 339 
crowded with capillaries with exceedingly thin walls. This very 
remarkable method of surface respiration is proved to exist in 
the nudibranch or shell-less marine Gasteropoda, for they possess 
neither gills nor lungs. 
OSPHRADIUM 
Lying just under the gills, and side by side with them, is a 
smaller, plume-like organ having the appearance of a smaller gill; 
it is called the osphradium. The office of this organ is not defi- 
nitely known, but the supposition is that it is the seat of the 
olfactory nerves. The osphradia are not always present, but when- 
ever found they are placed in close conjunction with the gills, in 
order, probably, that the animal may best determine the quality 
of the water brought by the siphon to the breathing-organs. In 
Buccinum the osphradium is especially prominent. In all mol- 
lusks the sense of smell is highly developed. A dead animal 
anchored to the bottom will very soon attract the carnivorous 
Mollusca from every direction. This is not a bad way to trap 
certain species that live upon rocky stations and are otherwise 
difficult to secure by the dredge. 
On the left-hand side (right of the animal), and adhering to 
the under side of the mantle flap, which has been thrown back, a 
somewhat inflated and convoluted tube, the rectum, will be seen. 
It terminates in an opening just within the mantle-edge. By 
the side of this are the renal and genital openings. 
Having now examined the external organs, we may investigate 
the internal anatomy of our subject. To do this thoroughly re- 
quires considerable skill, but for the purpose of merely discover- 
ing the relative positions of the principal internal organs a little 
care only is necessary. The work will be greatly facilitated by 
boiling the animal for a few minutes, for this serves to harden 
the tissues and to separate the various organs in a most satis- 
factory manner. 
To begin, open the proboscis from the mouth, cutting back so 
as to expose the esophagus. It will be seen that the throat is 
long and very muscular. 
