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 ospliradium. 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 ai*e 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 ospliradium 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. 



