MOLLUSCA. 129 



Class IV. Cephalopoda (Siphonopoda). 



Marine ; fore-foot united with head and forming tentacles or arms 

 surrounding the mouth ; mid-foot funnel-like ; sexes separate. A large 

 visceral hump and a mantle cavity containing two or four ctenidia and 

 the anal, genital, and paired nephridial apertures. Chromatophores in 

 the skin. 



Order 1. Tetrabranchiata, two pairs of ctenidia and nephridia, numerous 

 sheathed tentacles without suckers, and an external chambered shell. 

 Funnel incompletely closed ; no ink-bag. 



Nautili dse, septa of the shell concave, siphon central : Orthoceras 

 and Phragmoceras, fossil ; Nautilus pompilius, pearly nautilus, recent. 



AmmonitidaB, septa much folded at their edges, siphon external ; 

 all fossil : Goniatites, Ammonites (with aptychus and anaptychus), 

 Ceratites nodosus, Baculites, Toxoceras, Hamites. 



Order 2. Dibranchiata, a single pair of ctenidia and nephridia, eight or 

 ten arms covered with hooks or suckers, a completely closed funnel and 

 an ink-bag. 



Decapod a, ten arms, two of them much elongated ; suckers stalked 

 and with a horny ring. Body with two lateral fins, shell internal: 

 Belemnites (fossil). Spirula. Sepia officinalis, cuttlefish. Loligo 

 vulgaris, squid. Sepiola vulgaris. Onychoteuthis. 



Octopoda, eight arms, suckers unstalked and without a horny ring, 

 no internal shell, mantle cavity without a cartilaginous closing apparatus. 

 Octopus vulgaris, Eledone moschata. Argonauta argo, paper nautilus; 

 the female large, with a thin boat-shaped shell, secreted by the two dorsal 

 arms ; male small, without shell, and with one arm hectocotylized. 



