CEPHALOPODA 



607 



SUBORDER 1. DECAPODA. 



Body either elongate or more or less globose, with a pair of fins at 

 or near the aboral end, and with 10 arms, 4 pairs of shorter arms with 

 suckers along their entire length, and 1 pair of long 

 tentacular arms, often more or less retractile, with the 

 suckers confined to a broad terminal portion; suckers 

 pedunculate and with a chitinous supporting ring; shell 

 (Fig. 953) always present, and either chitinous or cal- 

 careous, and consisting, in its typical form (in the fossil 

 Belemnitidae), of a solid, conical, terminal rostrum and a 

 chambered conical phragmocone, which corresponds to the 

 shell of Nautilus, and is contained in the hollow base of 

 the rostrum, and whose broad, flat, anterior portion is 

 called the proostracon: about 250 species, grouped in 3 

 divisions; about 21 species on the Atlantic coast. 



Key to the divisions of Decapoda: 



a x Shell calcareous. 



& Shell spiral, with a siphuncle 1. PHRAGMOPHOBA 



6, Shell not spiral 2. SEPIOPHOBA 



a, Shell chitinous 3. CHONDBOPHOBA 



3 



Fig. 953 



Diagram of 



a belemnite 



shell (Leunls). 



1, proostra- 

 con ; 2, phrag- 



mocone ; 

 3, rostrum. 



DIVISION 1. PHRAGMOPHORA. 



Shell consists of a phragmocone, the septa of which are pierced by 

 a siphuncle near the inner margin of the whorl, with or without rostrum 

 and proostracon ; arms with hooks or suckers : 2 families, in one of which, 

 the fossil Belemnitidae of the Mesozoic age, the shell was 

 formed of the 3 typical parts (Fig. 953). 



FAMILY SPIEULIDAE. 



Shell partly internal and partly external, consist- 

 ing of a loose spiral with 2 or 3 coils, which is the 

 phragmocone alone; siphuncle contained in a continuous 

 calcareous canal: 1 genus. 



SPIRULA Lamarck. With the characters of the fam- 

 ily: 1 species. 



S. peroni Lam. (Fig. 954). Shell white and pearly, 25 mm. in diam- 

 eter; body red with brown spots, 55 mm. long, with a terminal sucker: 

 in deep water in tropical seas; the animal has been seen in only a very 

 few cases but the shells are common in the Caribbean Sea, and are occa- 

 sionally thrown up on the beach on Nantucket and other places along the 

 Atlantic coast. 



Fig. 954 The 

 shell of Spirula 

 peroni (Cam- 

 bridge Natural 

 History). 



