Page:1902 Encyclopædia Britannica - Volume 27 - CHI-ELD.pdf/356

 C U T T L E-F I S H CLASSIFICATION OF CUTTLE-FISH AND THEIR

320

Spirula.

Sepia.

Nautilus.

Onychoteuthis. Loligo. Oigopsida:. Myopsidse. DIBRANCHIATA.

Itetrabranchiata.

Spirulirostra. Sepia. Belosepia.

Aturia.

Plesioteuthis.

Belemnitella. Actinocamax. Belemnites.

Conoteuthis. Coccoteuthis. Plesioteuthis. Sepiophora. Acanthoteuthis. Belemnoteuthis. Geoteuthis. Teuthidse.

Xiphiteuthis. Belemnitidse.

Chondrophora.

Phragmoteuthis. Aulacoceras. PlIRAGMOPHORA. Orthoceras. DECAPODA. BELEMNOIDEA. U"

Sg ^c3 ^O t. ^ (li0> Cd Oc

Poterioceras.

Discites. Gyroceras.

Actinoceras.

LI PO PROTOCONCH I A. Gyroceras. Phragmoceras. /j'Ascoceras. lidfe. 4- Lituites.

t Trochoceras. Cyrtoceratidse.

Orthoceratidae^

HAPLOSIPHONIDA. Orthoceras. Cyrtoceras. l

Actinoceratidse.

Bactrites.

Piloceras. Endoceratidse.

METASIPHONIDA. Note.—This chart is to he read as if placed on a cylinder,

NAUTILOIDEA.

only covers the chamber adjacent to it, but afterwards grows over the sides and coalesces below. At the base is a flat depressed disc, which is not a sucker, but may be a pad to prevent shocks in swimming backwards. At the sides hang two fins which are parallel to each other, in the direction from back to front and not in one transverse line behind. The ovary lies against the right-hand side of the coil of the shell and the oviduct against the left, and the last chamber is occupied by part of the liver. The eyes are on peduncles sunk below the level of the head, but are not covered, as in higher groups. The Spirula thus represents an early stage in the development of the Order. The shells of the Cuttle-fish Order, on which we principally depend for our knowledge of their relations to fossil forms, are not in great variety at. the present day, but Extinct |]ie ftpirula-vhaW itself suffices to connect them farms. ^ the one pand witll tpe Pearly Nautilus, and on the other with an infinite series of extinct allies.

The

Spirula, like the Nautilus, has its shell in the form of a conical tube, coiled in one plane and divided by cross partitions the “ septa ”—into chambers connected by an inner tube the “ siphuncle.” Fossils showing these same general characters can be traced back to the earliest fossiliferous or Cambrian strata. With these, as the primitive ancestors, the history of the Cuttle-fish commences. These earliest forms of shell are, as might be expected, very simple. The tube is oval or round, the coiling is slight {Cyrtoceras), ov absent (Orthoceras), the septa mere shallow basins, the siphuncle a simple narrow tube with no fixed position, and the ornaments no more than lines of growth. In the later history we find one after another of the several elements becoming a centre of variation, till fixity is reached, or the element, or even the shell itself, is dispensed with. The first element to. vary was the siphuncle, the complexity of which gave rise to such early genera as Endoceras, Piloceras, and Actinoceras, forming a separate group, Metasiphoniba, the remain er