MOLLUSCA 



255 



SUBCLASS II. DIBRANCHIATA 



The Dibranchiata (Gr. Si?, two, and fipdyxia, gills) arc Cepha- 

 lopoda with only two gills. The portion of the foot surround- 

 ing the mouth is divided into eight or ten arms, which beai 

 from one to four rows of suckers, and the funnel is a completely 

 closed tube, open only at each end. The shell is usually in- 

 ternal, and when external is never chambered. The members 

 of this group are provided with an ink sac. The more striking 

 or interesting individuals may best be considered under the two 

 orders into which this subclass is generally divided, the classi- 

 fication depending on the number of arms surrounding the 



mouth. 



Order 1. Decapoda 



The Decapoda (Gr. Se/ca, ten, and 77-01;?, foot) have eight 

 sucker-bearing arms of nearly equal length, and two, much 

 longer, tentacle-like arms, with suckers only 

 near the tip ; the suckers are cup-shaped 

 and borne on little stalks. The body of 

 the animal is generally elongated, often to 

 a very considerable extent, and is provided 

 with a pair of lateral outgrowths or fins. 

 A shell is always present in the Decapoda ; 

 in one genus, Spirula(Fig. 264), it is spirally 

 coiled but loosely, and is partially cov- 

 ered by the mantle ; in all other genera it 

 is flattened, consisting of a single plate or 

 lamella and is internal — entirely inclosed 

 by the mantle. In the genus Sepia, the 

 cuttlefish (Fig. 260), this shell is calcareous 

 and constitutes the cuttlebone. The cuttle- 

 fishes live near the shore and feed on Crus- 

 tacea, Mollusca, and fishes ; they are caught 

 extensively and in some countries are eaten. 



In the genus Loligo (Fig. 265), to which 

 the common squids belong, the shell is en- 

 tirely chitinous ; it is commonly called the 

 pen of the squid because of its shape. In this country squids 

 are used very extensively for bait, particularly in the cod fish- 



FlG. 264. S/>,i ula peronii, 

 lateral d. terminal 



projecting portions of 

 shell, the internal portion 

 being shown in dotted 

 lines. (From Parker and 

 Haswell's Man 



