THE CEPHALOPODA 343 



TRIBE 2. TRACHYGLOSSA. 



These are Octopoda with a radula and without true fins. 



FAMILY 1. AMPHITRETIDAE, Hoyle. The funnel is attached to the 

 middle line of the mantle, dividing the pallial aperture into two. The 

 eight arms are united by a membrane. Genus Amphitretus, Hoyle ; 

 pelagic. FAMILY 2. ALLOPOSIDAE, Verrill. All the arms united by a 

 membrane. The mantle is joined to the head by a dorsal band and two 

 lateral commissures. Genus Alloposus, Verrill ; pelagic. FAMILY 3. 

 OCTOPODIDAE, d'Orbigny. Arms long and equal, without a true inter- 

 brachial membrane. The hectocotylus is not caducous. No cephalic 



Fio. 301. 



Argonauio. argo, Linnaeus, left side of the female. I, funnel ; II, mantle; III, eye; IV, 

 dorsal webbed arm. (After Verany.) 



aquiferous pores. Genera Octopus, Lamarck ; the suckers in two rows 

 on each arm ; British. Eledone, Leach ; a single row of suckers on each 

 arm ; British. Scaeurgus, Troschel. Pinnoctopus, d'Orbigny. Cistopus, 

 Gray. Japetella, Hoyle. FAMILY 4. PHILONEXIDAE, d'Orbigny. Males 

 and females naked. The hectocotylus is autotomous. The arms are un- 

 equal in size but similar in the two sexes. Aquiferous pores are present 

 on the head and funnel. Genera TremoctopVA, Delle Chiaje ; the two 

 dorsal pair of arms are united by a membrane. Ocythoe, Rafinesque ; 

 without an interbrachial membrane (Fig. 287). FAMILY 5. ARQO- 

 NAUTIDAE, Cantraine. The hectocotylised arm autotomous. The ex- 

 tremities of the dorsal arms are enlarged in the female (Fig. 301), and 

 secrete a shell in which the body is contained. The males are very small 

 and naked. Genus Argonauta, Linnaeus. 



