CEPHALOPODS. 



333 



tubercles. Argonauta hians is the typical species of the former group, and A. 

 navicula and A. tuberculata represent the latter. 



A, male argonaut (twice nat. size), WITH hkctocotylus enclosed in the sack ; B, MALE argonaut, with 



HECTOCOTYLUS FURTHER DEVELOPED. 



Suborder Decapoda. 



The Decapods form the second division of the two-gilled order, and differ from 

 the Octopods, as the name implies, by the possession of ten, instead of eight, arms. 

 The two additional arms differ from the rest in their greater length, and in having 

 suckers only at the extremity. They are frequently completely retractile within 

 pouches, and are used as prehensile organs in the capture of their prey. All 

 decapods are provided with an internal shell. That of the living species is either 

 horny — the so-called pen (gladius) ; or else calcareous — the bone (sepion) of the 

 cuttle-fish. In Spirula the shell takes the form of a tube, beautifully coiled, and 

 divided off into numerous air-chambers by a series of septa or partitions. The arms 

 of the decapods are furnished with pedunculated suckers, armed with horny rings 

 or hooks. The head is invariably distinct from the body. The eyes are free and 

 movable, and either covered with a fixed, transparent lid or skin, or unprotected 

 and in immediate contact with the water. All the species have either lateral or 

 posterior fins, and the funnel is provided with an internal valve. They live for 

 the most part out at sea, but some — Sepia, for example — are met with nearer the 

 shore. The pelagic forms are often found in immense shoals, and are eaten in 

 enormous quantities by many cetaceans and large fishes. When pursued by their 

 enemies, squids have been known to dart out of the sea with such force as to fall 

 upon the deck of a passing vessel. Decapods may be classified in three sections, 

 according to the character of the shell, of which the different types have already 

 been mentioned. 



