Page:The New International Encyclopædia 1st ed. v. 04.djvu/505

* CEPHALOPODA. 431 CEPHALOPODA. banks, that often athtiii-^ a linjrtli ovi-r all of 50 feet, of which the body has a lengtli of 9 feet, with a thickness of G feet, ami the arms the enormous length of 40 feet. This creature is indeed the 'devil-tish' about which so many thrilling stories have been told. Fossil- Cepiialoi'01>.. T!ie shells of cephalo- pods are more or less common in the rocks of all ages, from the top of the Cambrian to the top of the Mesozoic, while in the Tertiary deposits they are less frequently met with. In some formations they arc so abundant as to constitute the greater part of the mass. They are objects of much interest to the geologist, in that they furnish excellent 'indcv fossils' for the identifica- tion of the formations, as in the case of the Triassic and Jurassic rocks of Europe, wliidi are subdivided into a great number of zones, accord- ing to the particular species of Ammonoidea that they contain. To the paleontologist they are also" of very great interest, because, better than any other "class of animals, they allord op- portunity for studying the mode of evolution of races of "animals, and illustrations of the various laws of bioplastology, such as Agassiz's law of recapitulation or palingenesis, the inheritance of acquired characters, the laws of acceleration, etc. For some of the interesting and important resvilts derived from the study of the fossil shells of tetrabranchiate cephalopoiis, the reader should consult the papers of Hyatt, Buckman, Foord, and others, which are cited at the end of this article. The philosophic bearings of the study of eephalopods can be dwelt upon only lightly in this article, as lack of space prevents a proper array of facts necessary for their elucidation. The cephalopods were very important mem- bers of the faunas that inhabited the early seas of our globe, and were then far more abvindant than they ;ire now. The tetrabranchiates are represented bj- at least 7500 fossil species, of which number 5000 are members of the totally extinct order Ammonoidea. which began in the Silurian and culminated in the Cretaceous time. The remaining 2500 are Xautiloidea, which ap- jicared in the late Cambrian time, and which have continued to the present day, though they enjoyed tluir climax during the early Paleozoic ages of the Ordovician and Silurian. The di- branchiates are far less important as fossils. They appeared in the early ilesozoie, where they were represented by numerous Bclemnitoidea, which declined with the close of Cretaceous time, and they have continued to the present era, when they are represented by some 200 known spe- cies of the Sepioidea and Octopoda. The study of fossil cephalopods is based almost entirely on the shell characters, for traces of the soft parts are only seldom found. The fossil ink-bags, jaws, and hooks of certain dihranehi- ates are rare exceptions. (See Squid; Sepia.) As already indicated, cephalopods are divided, according to their gill-structure, into two sub- classes — Tctrabranchiata and Dibranchiata — which groups differ also in respect of their shell characters. The shell of the tetrabranch is built on the plan of that of the nautilus, though, in- stead of being closely coiled, it may be loosely coiled ((iyroccras), curved (Cyrtoceras), or straight (Orthoceras) . In the dibranchiates the shell is internal, and has the form of a [len or cuttlefish-bone: or. in those early Mesozoic gen- era that show dose relation.s to the tetrabranchs from which they have been recently evolved, the shape of a solid, limy, cigar-formed guard, into the larger end of which is inserted a chambered conical shell that resembles Orthoceras. (See liKT.KMNlTES. ) Oiic modern dibranchiate (Spi- rula) has an internal shell remarkably like tlial of certain loosely coiled nautiloiils of the De- vonian. The type of tetrabranchiate shell is found in the nautilus. Here the shell is coiled so closely that the outer wliorls to a considerable extent envelop or clasp the inner turns. In section, a nautilus-shell is seen to be essentially a coiled, elongated cone with quite regular transverse walls or 'septa' dividing the intenuil cavity into a number of chambers or 'cameras' of which the largest is the outermost or living-chamber occu- l)ied by the animal, lixtending from the living- chamb'er through all the posterior chambers to the apex of the shell, and piercing each septum, is a slender tube, the 'siphuncle,' which occupies a position near the centre of the cone, and w-hich serves to n:aintain communication between the living-chamber and the ]>osterior regions of the shell. Each of the septa represents a stage in the development of the individual, so that in each nautilus - shell we have all the stages through which the shell lias passed from the embryonic period, represented by the minute ape.x in the centre of the coil, to the adult or senile stage, represented by the living-chamber on the outside of the coil. It will be readily recognized that this condition is of utmost value for the study of the evolution of the group, and that it is scarcely equaled in any other class of animals. For this reason the cephalopods have furnished greater contributions to the knowledge of bio- plastology than have any other (Uganisms. At the apex of the nautilus-shell is seen a scar that marks the place of attachment of a deciduous embryonic shell or 'protoconch,' which probably fell otl' as soon as the true shell began to develop. The developmental stages of the living nautilus are less well known than are those of most of its extinct ancestors, but the existence of the deciduous embryonic sac is in- ferred from the presence of a thin-walled cal- careous bulb on the apices of several Paleozoic relatives of Nautilus, and also in its descend- ants, the goniatitoids and ammonoids. The septa are united to the side walls of the cone along lines that show prominently in the fossil forms, which are nearly always filled solid with infiltrated calcite. These are the suture-lines, which furnish characters of the greatest impor- tance in detei'iiiining the relationships, especially of the species of Ammonoidea. The lines of growth on the outside of the shell indicate the form of the margin of the aperture, and vary with the habits of the animal. In living Xau- tilus the hyponome is near the outer ventral curve of the ajiertiire of the shell, and there is on that side an emargination or sinus known as the 'hyponomie sinus.' The size of this sinus corresponds to the swimming activity of the animal: the larger the sinus the more active the swimmer, and vice versa. The aperture of Nau- tilus is open wide, and the animal is considered to be a good crawler. In some of the Paleozoic nautiloids the aperture is much restricted, and the ability to crawl must have been greatly di- minished, while the swimming power was in-