Phylogeny and Classification.—The order is divided into two sub-orders, Decapoda and Octopoda, by the presence or absence of the tentacular arms. The Decapoda are more adapted for swimming than the Octopoda, the body being usually provided with fins. In the former also there is generally an internal shell of considerable size, often calcined, while in the Octopoda only the merest vestiges of a shell remain. There can be no doubt that the Octopoda were derived from the Decapoda, although from the absence of skeletal structures fossil remains of Octopods are almost entirely unknown. Palaeoctopus, however, occurs in the Cretaceous, while shells of Argonauta do not appear before the Pliocene. The Decapoda are abundantly represented in the Secondary formations by the Belemnitidae, whose shell (fig. 19) consists of a straight conical phragmacone covered posteriorly by a very thick rostrum, and produced anteriorly into a thin long proöstracum which is only occasionally preserved. In certain cases remains of the arms provided with hooks, and of the ink-sac, have been recognized. The Belemnitidae appear first in the Upper Trias, attain their maximum development in the Jurassic rocks, and are not continued into the Tertiary period, though represented in the Eocene by a few allied forms.
There is no difficulty in deriving the typical existing Decapoda from Belemnitidae, and many of the extinct forms may have been directly ancestral. Chitinous “pens” like that of Loligo, however, begin to appear in the Jurassic and Cretaceous rocks, so that in this case as in many others the parent form and the modified form existed contemporaneously, and the latter alone has survived. The oldest shells of the Sepia type are from the Eocene, and it is perhaps possible that the Sepiidae arose separately from the Belemnites.
It is a curious fact that no fossil specimens of the genus Spirula have been found, but this may be due to the fact that it occurs only in deep water. At any rate there is no evidence that the shell of Spirula has lost a rostrum and a proöstracum; its characters must be regarded as primitive, not secondary. In the characters of the protoconch and of the commencement of the siphuncle, the shell of Spirula agrees with that of the Ammonoids, and in both its position is ventral, although in most Ammonoids the shell being exogastric the ventral side is the convex or external, while in Spirula the shell is endogastric and the siphuncle internal. The fact that the shell is not completely enclosed by the mantle is also a primitive character.
With regard to the general morphology of the Cephalopoda, it is difficult to reconcile the existence of two pairs of renal tubes as well as a pair of genital ducts in Nautilus with the view that the original Mollusc was unsegmented and had only one pair of coelomoducts. Considering the great specialization, however, and high degree of organization of the Cephalopods, it is evident that the earliest Nautiloid whose remains are known to us must have had a long evolutionary history behind it, and such metamerism as exists may have been developed in the course of its own history. In the other direction the evidence seems to prove that the Dibranchiata with only two renal ducts have been derived from the Tetrabranchiata.
Suborder 1. Decapoda.—Four pairs of ordinary non-retractile arms which are shorter than the body, and one pair of tentacular arms, situated between the third and fourth normal arms on each side and retractile within special pouches. Suckers pedunculated and provided with horny rings, on the tentacular arms confined usually to the distal extremities. Usually a well-developed internal shell, and lateral fins on the edges of the body. Heart in a coelomic cavity; nidamentary glands usually present.
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| Fig. 37.—Right and left sections through embryos of Loligo.(After Lankester.) | |
A, Same stage as fig. 35 (4). B, Same stage as fig. 35 (8);only the left side of thesections is drawn, and thefood-material which occupiesthe space internal tothe membrane ym isomitted. al, Rectum. is, Ink-sac. ep, Outer cell-layer. mes, Middle cell-layer. ym, Deep cell-layer of fusiformcells (yolk-membrane). ng, Optic nerve-ganglion. ot, Otocyst. | wb, The “white body” of theadult ocular capsule formingas an invagination ofthe outer cell-layer. mtf, Mantle-skirt. g, Gill. ps, Pen-sac or shell-sac, nowclosed. dg, Dorsal groove. poc. Primitive optic vesicle, nowclosed (see fig. 34). l, Lens. r, Retina. soc, Second or anterior opticchamber still open. if, Iridean folds. C, The primitive invaginationto form one of the otocysts,as seen in fig. 35 (5) and(6). |
Tribe 1. Oigopsida.—A wide aperture in the cornea. Two oviducts in the female. In fossil genera and Spirula, shell has a multilocular phragmacone with a siphuncle; initial chamber globular and larger than the second chamber. The most ancient forms characterized by the small size of the rostrum and proöstracum, and large size of the phragmacone. In the living genera, except Spirula, the shell is a chitinous gladius.
Fam. 1. Belemnoteuthidae. Extinct; shell with well-developed phragmacone, and rostrum merely a calcareous envelope; siphuncular necks directed backwards as in Nautiloidea; ten equal arms provided with hooks. Phragmoteuthis, Trias. Belemnoteuthis, Jurassic and Cretaceous. Acanthoteuthis, Jurassic.
Fam. 2. Aulacoceratidae. Extinct; phragmacone with widely separated septa; rostrum well developed and claviform. Aulacoceras, Trias. Atractites, Trias and Jurassic. Xiphoteuthis, Lias.