30 A MANUAL OF ZOOLOGY 



A crenated fold of membrane will be found just 

 behind the eyes, the olfactory organ. ' What is its extent? 



Notice the large triangular fin on the end of the body 

 farthest from the head. Consider its use. 



SURFACE — Dark brown pigment spots, chromato>- 

 phores, will be found on the surface. These vary in shape, 

 arid number. Observe, with the hand lens, that some of 

 .these have blurred edges and others are clear cut. The 

 chromatophores are very changeable during life. When 

 the animal was killed the chromatophores were caught in 

 the condition and shape in which they were at the time. 

 The changes are interesting to observe for the chromato- 

 phores gradually fade in color and reappear so evenly that 

 thtsy present a blush of color gradually from one point 

 to another over the entire surface. At times the color 

 almost disappears and the animal is nearly invisible in 

 the watfer. 



PLACE IN ANIMAL KINGDOM— Does the external ap- 

 pearance suggest that' this animal should belong in the 

 same class as the clam? Explain. 



Taking the DORSAL view of the head, is the animal 

 bilaterally symmetrical? If so how many arms are found 

 on each side? Are all the arms the same length? How 

 many rows of suckers are found on the short arms? On 

 the long arms? Draa -lfce dorsal viaw. 



Ventral View— What is the direction, of the muscular 

 bands that extend from the body to the fins? Notice the 

 projections of the anterior edge of the mantle. Find the 

 siphon projecting from under the mantle in the mid- 

 ventral line. What is its shape, size, and the direction 

 of its opening? Discuss the siphon as an organ of motion. 



Examine one of the grasping arms (long arms). Ex- 

 amine closely one of the suckers on this arm. Does it 

 have a stalk or is it sessile? Is there anything peculiar 

 about the margin of the suckers or acetabula? Cut through 

 the sucker and "piston longitudinally. Note carefully and 

 make draw4Bg-o£_t6e section. 



INTERNAL ST^tlTURB— Is the mantle fastened to 

 the head in any way? If so athow many points? Notice 

 mantle cartilage on the dorsal surface <of the iieck. What 

 is its shape and extent? Are^bere any projections from 



