84 BRANCH MOLLUSCA 



of a lung-sac instead of by gills, and must come to the surface occasionally 

 for air. In genus Physa the spiral of the shell is left-handed; in Limnte'a, 

 right-handed, and in Planor'bis the shell is discoid or a flat spiral. 



The eggs of genus Physa are deposited in gelatinous, transparent, 

 oblong capsules of an inch or less in length attached to submerged sticks 

 or leaves. Genus Limnc^a lays the eggs late in spring in capsules sur- 

 rounded by a mass of jelly. The young pass through a metamorphosis. 



Still other pond or river snails breathe by means of gills. They live in 

 the bottom of ponds or streams and are carnivorous. 



CLASS m. CEPHALOPODA 



Class Cephalop'oda (head-footed) consists of such forms as the 

 squid, cuttle-fish, octopus, and nautilus. They are all marine, 

 and, in many respects, the most highly developed of all mollusks. 

 There is a distinct head, bearing a pair of large well-developed 

 eyes, and surrounded by arms or tentacles which are modifica- 

 tions of the anterior margins of the foot. 1 The posterior part of 

 the foot is transformed into a funnel-like siphon. 



The body is bilaterally symmetric. Respiration is through 

 gills which line the mantle cavity. The shell may be external, 

 as in the nautilus; or internal, as the pen of the squid; or lacking, 

 as in the octopus. 



They are usually carnivorous. Some are solitary, as the 

 devil-fish; others, as the squid, go in immense shoals. The 

 senior author has seen acres of ground covered with the 

 catches of them on the Pacific coast. 



The circulatory system is closed and consists of a somewhat 

 complete heart and arteries, capillaries and veins. 



The principal ganglia are grouped about the esophagus. 

 The nervous system is the most highly developed of any of the 

 branch, consequently they are the most intelligent of all mol- 

 lusks. 



They have the power of quickly changing color to harmonize 

 with their environment. 



Cuttlefishes are rapid-swimming Cephalopoda living at a depth of several 

 fathoms, but sometimes coming into shallower water. The cuttlefish has a 

 distinct head bearing ten long arms, and a pair of highly developed eyes 

 resembling those of a fish. The free end of the head bears the mouth. 

 The inner surface of each arm or tentacle is flat and bears four longitudinal 

 rows of suckers. The fourth pair of tentacles is much longer and more 

 slender than the others, and the club-shaped end bears suckers. The 



1 See McMurrich, p. 341. 



