MOLLUSCA 



135 



no external shell covering the body, but usually a horny or 

 calcareous part within. They have a distinct head, promi- 

 nent eyes, horny mandibles, eight or ten arms furnished 

 with suckers, two gills, a complete tubular funnel, and an 

 ink bag containing a peculiar fluid (sepia}, of intense 

 blackness, with which the water is darkened to facilitate 

 escape. They have the power of 

 changing color, like the chame- 

 leon. They crawl with their arms 

 on the bottom of the sea, head 

 downward, and also swim back- 

 ward or forward, usually with the 

 back downward, by means of fins, 

 or squirt themselves backward by 

 forcing water forward through 

 their breathing funnels. ; - 



The paper nautilus (Argonautd) 

 and the poulpe (Octopus) have eight 

 arms. The female argonaut se- 

 cretes a thin, unchambered shell 

 for carrying its eggs. The squid 

 (Loligo) and cuttlefish (Sepia) have FIG. 108. 



1 -. -. . . , , 



ten arms, the additional pair be- 

 ing much longer than the others. 

 Their eyes are movable, while those 

 of the argonaut and poulpe are 

 fixed. The squid, so much used 

 for bait for cod, has an internal 

 horny "pen," and the cuttle has a spongy, calcareous 

 " bone." The extinct Belemnites had a similar structure. 

 Squid have been found with a body eleven feet and 

 arms thirty-nine feet long, and parts of others still 

 larger as much as seventy feet in total length. 



2. Tetrabranchs. This group is characterized by the 

 possession of four gills, forty or more short tentacles, 



y eatti) with 

 the mantle cut open : b, bran- 

 chial heart ; e, eye ; f, fin ; g, 

 gill; /', intestine; ib, ink bag; 

 m, cut edge of mantle; ma, 

 mantle artery; me, mantle 

 cavity; met, mantle carti- 

 lages; pvc, posterior vena 

 cava; s, siphon; t, tentacles 

 with sucking disks; us, visce- 

 ral sac. 



