CEPHALOPODA.] Octopoda they are not unfrequently connected by a web, and form an efficient swimming-bell. The suckers are placed on the ad-oral surface of the arms, and may be in one, two, or four rows, and very numerous. In place of suckers in some genera we find on certain arms or parts of the arms horny hooks ; in other cases a hook rises from the centre of each sucker. The hooks on the long arms of Onychoteuthis are drawn in fig. 97. The fore-foot, with its apparatus of suckers and hooks, is in the Dibranchiata essentially a prehensile apparatus, though the whole series of arms in the Octopoda serve as swimming organs, and in many (e.g., the Common Octopus or Poulp) the sucker- bearing surface is used as a crawling organ. In the males of the Dibranchiata one of the arms is more or less modified in connexion with the reproductive function, and is called the " hectocotylized arm." This name is derived from the condition assumed by the arm in those cases in which its modification is carried out to the greatest extent. These cases are those of the Octo- pods Argonauta argo and Parasira catenulata (fig. 96). In the males of these the third arm (on the left side in Argonauta, on the right side in Parasira) is" found before the breeding season to be represented by a globular sac of integument. This sac bursts, and from it issues an arm larger than its neighbours, having a small sac at its extremity in Parasira (fig. 96, x), from which subsequently a long filament issues. Before copulation the male charges this arm with the spermatophores or packets of spermatozoa removed from its generative orifice beneath the mantle-skirt, and during coitus the arm becomes detached and is left- adhering to the female by means of its suckers. A new arm is formed at the cicatrix before the next breeding season. The female, being much larger than the male, swims away with the detached arm lodged beneath her mantle-skirt. There, in a way which is not understood, the fertilization of the eggs is effected. Specimens of the female Parasira with the detached arm adherent were examined by Cuvier, who mistook the arm for a parasitic worm and gave to it the name Hectocotylus. Accordingly, the correspondingly modified arms of other Siphonopoda are said to be hecto cotylized. Steenstrup has determined the hectocotylized condition of one or other of the arms in a number of male Dibranchs as follows : in all, excepting Argonauta and Parasira, the modification of the arm is slight, consisting in a small enlargement of part or the whole of the arm, and the obliteration of some of its suckers, as shown in fig. 95, A, B; in Octopus and Eledone the third right arm is hectocotylized ; in Rossia the first left arm is hectocotylized along its whole length, and the first right arm also in the middle only ; in Sepiola only the first left arm along its whole length ; in Sepia it is the fourth left arm which is modified, and at its base only ; in Sepioteuthis, the same at its apex ; in Loligo, the same also at its apex ; in Loliolus, the same along its whole length ; in Ommastrephes, Onychoteuthis, and Loligopsis no hectocotylized arm has hitherto been observed. In the females of several Dibranchs (Sepia, &c.) the packets of spermatozoa or spermatophores received from the male have been observed adhering to the smaller arms. How they are passed in this case by the female to the ova in order to fertilize them is unknown. Musculature, Fins, and Cartilaginous Skeleton. Without entering into a detailed account of the musculature of Nautilus, we may point out that the great muscular masses of the fore-foot and of the mid-foot (siphon) are ultimately traceable to a large transverse mass of muscular tissue, the ends of which are visible through the integument on the right and left surfaces of the body dorsal of the free flap of the mantle-skirt (fig. 89, I, I, and fig. 91, &). These muscular areas have a certain adhesion to the shell, 675 and serve both to hold the animal in its shell and as the fixed supports for the various movements of the tentaculi- ferous lobes and the siphon. They are to be identified with the ring-like area of adhesion by which the foot-muscle of the Limpet is attached to the shell of that animal (see fig. 27). In the Dibranchs a similar origin of the muscular masses of the fore-foot and mid-foot from the sides of the shell modified, as this is, in position and relations can be traced. In Nautilus there are no fin-like expansions of the integu ment, whereas such occur in the Decapod Dibranchs along the sides of the visceral hump (figs. 92, 93). As an excep tion among Octopoda lateral fins occur in Pinnoctopus (fig. 94, A), and in Cirrhoteuthis (fig. 94, D). In the Ptero- podous division of the Cephalopoda such fin-like expansions of the dorsal integument do not occur, which is to be con nected with the fact that another region, the mid-foot, which in Siphonopods is converted into a siphon, is in them expanded as a pair of fins. In Nautilus there is a curious plate-like expansion of integument in the mid-dorsal region just behind the hood, lying between that structure and the portion of mantle- skirt which is reflected over the shell. This is shown in fig. 90, b. If we trace out the margin of this plate we find that it becomes continuous on each side with the sides of the siphon or mid-foot. In Sepia and other Deca pods (not in Octopods) a closely similar plate exists in an exactly corresponding position (see b in figs. 110, 111). In Sepia a cartilaginous development occurs here immediately below the integument forming the so-called " nuchal plate," drawn in fig. 116, D. The morphological significance of this nuchal lamella, as seen both in Nautilus and in Sepia, is not obvious. Cartilage having the structure shown in fig. 117 occurs in various regions of the body of Siphono poda. In all Glossophorous Mollusca the lingual apparatus is supported by internal skeletal pieces, having the char acter of cartilage ; but in the Siphonopodous Cephalopoda such cartilage has a wider range. In Nautilus a large H-shaped piece of cartilage is found forming the axis of the mid-foot or siphon (fig. 116, A, B). Its hinder part extends up into the head and supports the peri-cesophageal nerve-mass (), whilst its two anterior rami extend into the tongue-like siphon. In Sepia, and Dibranchs generally, the cartilage takes a different form, as shown in fig. 116, C. The processes of this cartilage cannot be identified in any way with those of the capito- pedal cartilage of Nautilus. The lower larger portion of this cartilage in Sepia is called the cephalic cartilage, and forms a complete ring round the oesophagus ; it completely invests also the ganglionic nerve-collar, so that all the nerves from the latter have to pass through foramina in the cartilage. The outer angles of this cartilage spread out on each side so as to form a cup-like receptacle for the eyes. The two processes springing right and left from this large cartilage in the median line (fig. 116, C) are the " prse-orbital cartilages ; " in front of these, again, there is seen a piece like an inverted T, which forms a support to the base of the " arms " of the fore-foot, and is the " basi- brachial " cartilage. The Decapod Dibranchs have, further, the " nuchal cartilage " already mentioned, and in Sepia, a thin plate-like " sub-ostracal " or (so-called) dorsal cartilage, the anterior end of which rests on and fits into the concave nuchal cartilage. In Octopoda there is no nuchal cartilage, but two band-like " dorsal cartilages." In Decapods there are also two cartilaginous sockets on the sides of the funnel " siphon-hinge cartilages " into which fleshy knobs of the mantle-skirt are loosely fitted. In Sepia, along the whole base-line of each lateral fin of the mantle (fig. 92), is a " basi-pterygial cartilage." It is worthy of remark that
we have, thus developed, in Dibranch Siphonopods a more