1046 CEPHALOPODA. [Dibranchia. 



ORDER 2. DIBRANCHIA, Owen. 



In these Cephalopoda the external surface of the visceral mass 

 is naked, and is only protected by a more or less rudimentary shell, 

 which is situated on the aboral surface, and covered by the integu- 

 ments of this region. The female Aryonauta is the only member of 

 the group that has a wholly external shell, but this is not adherent, 

 and is secreted by the dorsal arms. The head of the Dibranchia 

 bears 8 acetabuliferous arms, and there is frequently a fifth pair of 

 more or less retractile arms, situated between the third and fourth 

 pair. The funnel is always a completely closed tube. There are 

 2 branchise and 2 kidneys, each of the latter having a pericardial 

 orifice. The cephalic cartilage is traversed by the oesophagus, and 

 encloses all the principal nervous centres. The ocular cavities are 

 closed, and the eyes have a crystalline lens. Chromatophores are 

 present in the integument, and an ink-sac is generally present. The 

 Dibranchia include 2 suborders, the Decapoda and the Octopoda. 



SUBORDER 1. DECAPODA. 



In this suborder, in addition to the 4 pairs of normal arms, there 

 is a more or less well-developed " tentacular " arm situated between 

 the third and fourth normal arms, on each side of the head. These 

 tentacular arms are more or less retractile within special pouches, 

 and, as a rule, they only bear suckers at their free extremities. The 

 suckers are pedunculated, and provided with horny rings. The 8 

 normal arms are shorter than the body. There is generally a fairly 

 well-developed internal shell, and there are usually lateral fins of 

 various width. The heart lies in a ccelomic cavity. Nidamentary 

 glands are usually present. 



Tribe 1. OIGOPSIDA. 



The members of this tribe are characterized by the presence of a 

 wide orifice, occupying the optic axis, in the external false cornea 

 of the eye. As a rule, 2 oviducts are present. With the exception 

 of Spirula, the shell is a chitinous gladius. The Oigopsida are the 

 most archiac of the Dibranchia. 



Fam. SPIRULIDJE, d'Orbigny. 



Animal : Body oblong, with minute terminal fins ; mantle sup- 

 ported by a cervical and 2 ventral ridges and grooves ; arms with 

 6 rows of minute cups, tentacular arms elongated ; siphon valved. 



Shell spiral, whorls on the same plane, not in connection, cham- 

 bered ; chambers connected by a ventral siphon, invested by a series 

 of cone-shaped tubes, 1 for each chamber. The shell is placed verti- 

 cally in the end of the body, and is held in place by side flaps of the 

 mantle ; the 2 dorsal and ventral sides of the aboraj extremity of the 

 shell are left uncovered bv the mantle. 



