GENERAL CHARACTERS OF CEPEALOPODS. 253 



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

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

 like fin. The sexes are distinct. Respiration by gills. Orr 

 der 1. Pterotracheidm. — Pterotrachea, Carinaria, Firoloides. 

 Order 3. Atlantidoe. — Atlanta (living) ; Bellerophon (fossil). 



Laboratory Work. — The Gastropods are very difiBcult 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 (Fig. 

 311), an iron frame with a 

 net, to ■which is attached a 

 rope and weight. 



Kg. 206.— DredgeJ 



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. 307. The following account 

 is based on dissections of Loligo Pealii Lesueur (Fig. 

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

 with its arms and suckers, is given in the accompanying 

 illustration (Fig. 207) 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 



