GENERAL CHARACTERS OF CEPUALOPODS. 273 



Subclass 4. Heteropoda. Naked or shell-bearing mollusks, with a large 

 prominent head, large movable eyes, and foot with a keel- 

 like fin. The sexes are distinct. Respiration by gills. Or- 

 der 1. Pterotracheidm. Pterotrachea, Carinaria, Firoloides. 

 Order 2. Atlantidce. Atlanta (living) ; Bellerophon (fossil). 



Laboratory Work. The Gastropods are very difficult to dissect, and 

 it is quite essential that the 

 specimen be freshly killed, and 

 that it has died as fully ex- 

 panded as possible. For this 

 purpose they should be al- 

 lowed, as Verrill suggests, to 

 die in stale sea-water, with 

 the parts expanded ; when the 

 animal is nearly dead, the soft 

 parts can be forcibly held out 

 by the hand while the animal 

 is killed by immersion in alco- 

 hol. Shells and other marine 

 animals may be obtained by 

 means of the dredge (Fitr. 

 211), an iron frame with a 

 net, to which is attached a 

 rope and weight. 



CLASS III. CEPHALOPODA (Squids and Cuttle-fishes). 



General Characters of Cephalopods. The essential 

 features of this class may be observed by a study of the com- 

 mon squid, represented by Fig. 212. The following account 

 is based on dissections of Loligo Pealii Lesueur (Fig. 

 213). A general view of the body of the entire squid, 

 with its arms and suckers, is given in the accompanying 

 illustration (Fig. 212) of Loligo pallida Verrill. The body 

 is fish-like, pointed behind, and with two broad fleshy fin- 

 like expansions at the end of the body. The head is dis- 

 tinct from the mantle or body, and the mouth is surrounded 

 by a crown of ten long stout pointed arms, provided on the 

 inner side with two rows of alternately arranged cup-shaped 

 suckers, each sucker being spherical, hollow, with a horny 



