336 MARINE INVERTEBRATES 



the ventral surface of the animal, giving to the creature, when 

 active, the appearance of crawling upon its stomach. 



If a living gasteropod is placed in a jar of sea- water and left 

 undisturbed it will soon crawl up the side of the glass and thus 

 afford an excellent view of the extended under surface of its foot. 

 The majority of the gasteropods are lethargic and slow in move- 

 ment,- but others will be found to be exceedingly lively and able 

 to creep up the sides of the jar and make their escape in a sur- 

 prisingly short time. Further modifications of the foot will be 

 mentioned later. 



THE MANTLE CAVITY 



Now let us place the animal with the foot below and the head 

 facing us. The spiral visceral hump will point back and away 

 from the observer. The mantle covering the visceral portion 

 is thin and translucent like an ordinary skin, but at the base 

 of the visceral mass the mantle suddenly thickens very greatly, 

 and spreads over the dorsal portion of the foot or the body like a 

 cloak. The space between the body and the loose-lying mantle 

 is called the mantle cavity, and within this area are to be found 

 several important external organs. To find these it is well to cut 

 the mantle flap in a straight line, beginning just back of the head 

 and ending at the point where the mantle is attached to the body. 

 This will most easily be done with scissors. Having thus divided 

 the mantle in front, throw back the two flaps. On the right-hand 

 side (left of the animal), and attached to the inner side of the 

 mantle thus exposed, will be seen the branchiae, or gills. 



THE BRANCHIAE, OR GILLS 



They consist of a row of flattened filaments bound together at 

 the base like the leaves of a book. The blood is conveyed to 

 these gills by a large vein, and is then forced through the thinly 

 walled filaments, being thus brought into close contact with the 

 water, to which it releases its carbon dioxide, and from which 

 it receives the life-giving oxygen. In certain gasteropods there 

 are two sets of gills, one placed upon each side of the body 



