138 



MOLLUSCA 



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 catenvlata (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. 



Mvsntlatvre, Fins, mid Ga/rtHaginous 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, !, !, and fig. 91, ). 

 These muscular are;e have a certain adhesion to the shell, 



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 (a), 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 

 " pra>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 



