externally. In this order the ducts of the generative organs open into the pallial chamber, and each individual, besides the essential organs of reproduction (testis or ovary), generally possesses an accessory gland; that of the female secreting a viscid material which unites the eggs together, whilst that of the male coats the spermatozoa, and aggregates them into peculiar worm-like filaments, from six to eight lines in length, termed "spermatophores," or the "moving filaments of Needham." The spermatophore is filled with spermatozoa, and possesses the power of expanding when moistened, rupturing, and expelling the contained spermatozoa with considerable

نا

2. Ven

Fig. 210.-1. Octopus carena (male), showing cyst in place of the third arm. tral side of an individual, more developed, with the hectocotylus (a). (After Woodward.)

force. During the congress of the sexes the male transfers the spermatophores to the pallial chamber of the female, true intromission not being possible, but the mode in which this transference is effected differs in different cases, and is not universally known.

In the males of many of the Cuttle-fishes, one of the arms is peculiarly modified, and is said to be "hectocotylised," but the extent to which this modification is carried differs in different cases, and it is not always the same arm in different species which is thus affected. In some cases, the "hectocotylised" arm is little altered from its ordinary form, and though the alteration be primarily sexual, the arm is not known to play any part in the reproductive process. In other cases, again, such as Octopus carena (fig. 210), Tremoctopus violaceus (fig 211, b), and Argonauta argo (fig. 211, a), the "hectocotylised"

Fig. 211.-a, Male of Argonauta argo, with the hectocotylised arm still contained in its enveloping cyst, four times enlarged (after H. Müller); b, Hectocotylus of Trema topus violaceus (after Kölliker.

arm is the efficient agent in the impregnation of the female. It is, in these forms, longer and thicker than the other arms, and possesses posteriorly a sac which is filled with spermatophores. During the reproductive act the "hectocotylised" arm is actually detached by the male, and deposited, with its freight of spermatophores, within the pallial chamber of the female. When thus detached (fig. 211, b), it is capable of independent movement, and when first found in this free condition within the mantle-cavity of the female Argonaut, it was regarded as a parasitic worm. Cuvier gave the name of "Hectocotylus Octopodis" to it, under this belief as to its nature. Hence the name of "hectocotylus" (in allusion to the suckers which it carries) is still applied to the detached arm;

whereas the arm, if not detached, is simply said to be "hectocotylised."

In those cases in which the hectocotylised arm is not detached, it is asserted by Steenstrup that it is employed by the male in the direct transference of the spermatophores to the pallial chamber of the female; though it is still uncertain how the spermatophores find their way from the seminal ducts to the sac in the interior of the arm.

The eggs of the Cuttle-fishes are enclosed, singly or many together, in special capsules, which are generally attached in bunches to some foreign body. The ovum undergoes partial segmentation, as in Birds and Reptiles, and the unsegmented portion of the yolk is gradually absorbed by the growing embryo.

The shell of the Cephalopoda is sometimes external, some

Fig. 212-a Internal skeleton of Sepia ornata; b Pen of Histioteuthis Bonelliana; Shell ("phragmacone") of Spirula fragilis; d Animal of Spirula Peronii.

times internal. The internal skeleton (fig. 212) is known as the "cuttle-bone," "sepiostaire," or "pen" (gladius), and may be either corneous or calcareous. In some cases it is rendered complex by the addition of a chambered portion or "phragmacone," which is to be regarded as a visceral skeleton or

"splanchnoskeleton." In Spirula (fig. 212, c) the phragmacone is the sole internal skeleton, and is coiled into a spiral, the coils of which lie in one plane, and are near one another, but not in contact. It thus resembles the shell of the Pearly Nautilus, but it is internal, and differs, therefore, entirely from the external shell of the latter. The only living Cephalopods which are provided with an external shell are the Paper Nautilus (Argonauta), and the Pearly Nautilus (Nautilus pompilius); but not only is the structure of the animal different in each of these, but the nature of the shell itself is entirely different. The shell of the Argonaut (fig. 213) is involuted, but is not divided into chambers, and it is secreted by the webbed extremities of two of the dorsal arms of the female. The arms are bent backwards, so as to allow the animal to live in the shell, but there is in reality no organic connection between the shell and the body of the animal. In fact, the shell of the Argonaut, being confined to the female, and serving by its empty apex as a receptacle for the ova, may be looked upon as a "nidamental shell," or as it is secreted by a modified portion of the foot, it may more properly be regarded as a "pedal shell." The shell of the Pearly Nautilus (fig. 216), on the other hand, is a true pallial shell, and is secreted by the body of the animal, to which it is organically connected. It is involuted, but it differs from the shell of the Argonaut in being divided into a series of chambers by shelly partitions or septa, which are pierced by a tube or "siphuncle," the animal itself living in the last chamber only of the shell.

CHAPTER L.

DIVISIONS OF THE CEPHALOPODA.

THE Cephalopoda are divided into two extremely distinct and well marked orders, termed the Dibranchiata and Tetrabranchiata. The former comprises all the true Cuttle-fishes; whilst the latter, though abundantly represented in past time, has no other living representative than the Pearly Nautilus alone.

ORDER I. DIBRANCHIATA.—The members of this order of the Cephalopoda are characterised as being swimmimg animals, almost invariably naked, with never more than eight or ten arms, which are always provided with suckers. There are two branchia,

which are furnished with branchial hearts; an ink-sac is always present; the funnel is a complete tube, and the shell is internal, or, if external, is not chambered.

The Cuttle-fishes are rapacious and active animals, swimming freely by means of the jet of water expelled from the funnel. The arms constitute powerful offensive weapons, being excessively tenacious in their hold, and being sometimes provided with a sharp claw in the centre of each sucker. They are mostly nocturnal or crepuscular animals, and they sometimes attain to a great size. They may be divided into two sections, Octopoda and Decapoda, according as they have simply eight arms, or eight arms and two additional "tentacles." SECTION A. OCTOPODA.-The Cephalopods comprised in this section are distinguish

ed by the possession of not more than eight arms, which are provided with sessile suckers. The shell is internal and rudimentary; in one instance only (the Argonaut) external. The body is short and bursiform, and ordinarily without fins.

This section comprises the two families of the Argonautida, and the Oc-. topodide. In the former of these there is only the single genus Argonauta (the Paper Sailor, or the Paper Nautilus), of which the female and male differ greatly from one another. The female Argonaut (fig. 213) is protected by a thin

single-chambered shell, in Fig. 213-Argonauta argo, the "Paper Nau

form symmetrical and involuted, which is secreted by the webbed extremities

tilus," female. The animal is represented in its shell, but the webbed dorsal arms are separated from the shell, which they ordinarily embrace.

of the dorsal arms, but is not attached in any way to the body of the animal. It sits in its shell with the funnel turned towards the keel, and the webbed arms applied to the shell. The male Argonaut is much smaller than the female (less than an inch in length), and is not protected by any shell. The