GENERAL CHARACTERS OF CEPHALOPODS. 2538 
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. Pterotracheidw.—Pterotrachea, Carinaria, Firoloides, 
Order 2. Atlantide.—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 (Fig. 
211), an iron frame with a 
net, to which is attached a Fig. 206.—Dredge. 
rope and weight, 
Cuass III.—CrEpHatopopa (Sguids 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. 20%. The following account 
is based on dissections of Loligo Pealit Lesueur (Fig. 
208). 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 
