Cephalopoda.- 



-MOLLUSCA.- 



-OCTOPODA. 



311 



enveloped in the mantle, which forms a muscular sac, 

 open only in front. The head protrudes from this 

 openinf^, is distinct from the body, and has placed 

 round the mouth a number of flexible, fleshy append- 

 ages, called arms or feet, which serve both as organs of 

 prehension and locomotion. These arms or feet are 

 usually provided with cups or suckers, called acetahula, 

 that act like cupping glasses, and are for the most part 

 unarmed, tliough in some instances they are furnished 

 with a long sharp claw. They form a single or double 

 series on the inner surfoce of the arms, and are various 

 in number. In those which are unarmed, the mechan- 

 ism for producing adhesion is extremely interesting. 

 From the margin of each cup, a series of muscular 

 fibres converge toward the centre, at a short distance 

 from which they leave a circular aperture, which is 

 occupied by a cone of soft substance rising from the 

 bottom like the piston of a syringe, and capable of 

 retraction. So perfect is this mechanism, says Dr. 

 Roget, that while the muscular fibres continue con- 

 tracted, it is easier to tear away the substance of the 

 limb than to release it from its attachment — it being 

 apparently under the complete control of the animal. 

 When the suoktr is applied to a surface for the pur- 

 pose of adhesion, this piston, having previously been 



raised so as to fill the cavity, is retracted, and a vacuum 

 produced ; but when the animal is disposed to let go 

 its hold, the piston is at once pushed forward, and 

 the vacuum produced by its retraction is in an instant 

 destroyed. 



The class Cephalopoda is divided into two orders — 

 those which have the body naked, and have no shell, 

 or only an internal one ; and those in which the body 

 is inclosed within an external chambered shell. The 

 first group, the Naked Cephalopods, have all of them 

 only two branchise, or gills ; whilst the second group, 

 the Shell-bearing Cephalopods, possess four. This 

 essential character has been considered as the best for 

 dividing them, as one of the families of the first group 

 {Argonaiitid(e) is now generally allowed to possess a 

 shell, though not a chambered one ; and another 

 (Sjiirulidce) has a chambered shell, which though cer- 

 tainly an internal one, nevertheless in one of the species 

 appears to contain a portion at least of the animal 

 within its last chamber. Rafinesque, in dividing them 

 info two groups or orders, proposed the names of 

 Antcpedia and Polarnaxia for them; but we hero adopt 

 the nomenclature first established by Professor Owen, 

 who from the number of their gills calls them Dihraa- 

 chiata and TeCrabranchiata. 



Order I.— DIBRANCHIATA— Two-gilled Cephalopods, 



Animal swimming, having the body naked, the shell 

 being either altogether wanting or contained within the 

 body (except in Argonautidce). The head is distinct, 

 separate from the rest of the body, and surrounded by 

 eight or ten fleshy arms, provided with sessile cups or 

 suckers. The eyes are sessile, prominent ; the mandi- 

 bles horny, like the beak of a parrot; and the walls of 

 the siphuncle are entire. Gills two in number. Ink 

 gland always present. The body of the animal is 



round, or elongated, and provided with fins ; and 

 when the shell is present (except in Argonauta), it is 

 either horny or shelly, with or without air chambers. 

 " The mantle is usually connected with the back of 

 the head by a broad muscular band ; but its margin 

 is sometimes free all round, and is supported only 

 by cartilaginous ridges, fitting into corresponding 

 grooves, and allowing considerable freedom of motion." 

 — ( Woodward.) 



SUB-OEDER I.— OCTOPODA (OCTOPODS), 



This sub-order contains three families. The body is 

 rounded, and seldom possesses fins ; it is united to the 

 head by a broad cervical band, and in the Argonauts 

 is covered with a thin, single chambered shell. The 

 head is large, and the eyes are fixed and incapable of 

 rotation. The fleshy arms are only eight in number, 

 and have no swimming membranes. The cups or 

 suckers are sessile, flat, and without any horny ring. 

 The branchial chamber is divided longitudinally by a 

 muscular partition. " They are the most eccentric or 

 'aberrau'' molluscs, superior in organization to all the 

 rest, but manilesting some remarkable and unexpected 

 analogies with the lowest classes of animals.'' — ( Wood- 

 vard.) 



\Ve have merely space to mention a few of the 

 families and genera into which this order is sub- 

 divided : — 



Familv. I.— OCTOPODID^— (ne Sea Spiders.) 



In this family the anns are all similar, subulate in 

 form, or conical and tapering, elongated, and united 

 at the base by a web. The cups are short and sessile. 

 The shell is altogether wanting, or is represented by 

 two short styles encysted in the substance of the mouth. 

 — {Owen.) 



Genus Octopus. — The genus Octopus may be 

 taken as the type of the family. It has two rows of 

 cups on the arms, and the body of the animal is round, 

 and destitute of lateral fins. It is warty, or covered 

 with cirrhi, which disappear when in repose, or when 

 preserved in spirits. The eyes are small, lateral, 

 covered by the continuation of the surrounding skin, 

 and often by one or two transparent eyelids. The 



