586 MOLLUSCA phylum vi 



tades Surround the moufh, and serve as prehensile and locomotive organs ; in the 

 Dibranchiates tliey are armed with hooJcs and sucker s.^ 



The Cephalopods are the most highly organised, and include the largest- 

 sized of all known Mollusca. They breathe by gills, and are exclusively marine. 

 Their nervous, circulatory, digestiye and reproductive Systems, their muscula- 

 ture and sense organs all exhibit remarkable differentiation as compared with 

 those of other Mollusks. A fleshy mantle, which is open above, encloses the 

 cavity which is occupied by the respiratory organs (the gills) and it also 

 serves as a covering for the reproductive, alimentary and secretory Systems, 

 the heart and the principal blood-vessels. A large ganglionic mass (cerebral 

 ganglion) and sub-oesophageal ganglion connected by commissures are placed 

 around the Oesophagus, and are surrounded by a cartilaginous enclosure in the 

 Dibranchiates, but in Nautilus this protects only the sub-oesophageal nerve mass. 



Eecent Cephalopods were divided by Owen into two groups — Tetrahranchiata 

 and Dibranchiata. The former is represented in the present fauna by the soli- 

 tary genus Nautilus, but the latter still comprises a very considerable series of 

 forms. A host of fossil Cephalopods abounded in the Paleozoic and Mesozoic 

 seas. Among these the two largest groups, the Ammonoidea and Belemnoidea, do 

 not afford any certain information regarding the number of gills, but the shells 

 of the former agree essentially with those of Nautili, while those of Belemnites, 

 on the other band, are more like those of certain Dibranchiates ; hence it is 

 advisable to associate these fossil groups with the corresponding sub-classes 

 established for Eecent forms. 



Subclass 1. TETRABRANCHIATA Owen.' 



Cephalopods with four plumose gillSj and external chambered shells. Amhula- 



Oberdevon am Enkeberg. Neues Jahrb. f. Min. etc., 1908, Supplem. vol. xxvi. — White, C. A., 

 Mesozoic Fossils. Bull. U.S. Geol. Surv. No. 4, ISSA.—Whüeaves, J. F., Mesozoic Fossils, 

 vol. i. Geol. Surv. Canada, 1876-79. — Palaeozoic Fossils, vol. iii., ibid. 1884-97. — Contributions 

 to Canadian Palaeontology, vol. i., 1885-89. — Descriptions of Fossils from the Devonian of Manitoba. 

 Trans. Roy. Soc. Canada, vol. viii. sec. 4, 1890. — Whitßeld, R. P., Several papers in Bull. Amer. 

 Mus. Nat. Hist., 1886-97.— Eepublication of Hall's Fossils, etc. Ibid. vol. i., pt. ii., 1895. 

 Wright, T., Monograph on the Lias Ammouites. Palaeont. Soc, 1878-86. — Würtenberger, R., 

 Studien über die Stammgeschichte der Ammoniten. Darwinistische Schriften, No. 5. Leipzic, 

 1880. — Vabe, H., Cretaceous Cephalopoda from the Hokkaido. Journ. Coli. Sei. Iniper. Univ. 

 Tokyo, Japan, 1904, vols. xix., xx. — Zittel, K. A., Cephalopoden der Stramberger Schichten. 

 Palaeont. Mittheil. Museum Bayer. Staates, Bd. ii., 1868. — Die Fauna der älteren Tithonbildungen. 

 Ibid. Bd. iii., 1870.— Handbuch der Paläontologie, Bd. iL, 1881-85. 



^ A. E. Verrill has furnished the following note regarding the arms of Cephalopods : " The 

 arms, together with the siphon (ambulatory funnel) of Cephalopods, must be considered as 

 homologous with the foot of other Mollusca. The large nerves supplying these organs arise from 

 the pedal ganglia. In the early larval stages the arms arise as bud-like, paired lateral outgrowths 

 at the base of the large yolk-sac, while the rudiments of the siphon (fxmnel) arise as two oblique 

 pairs of folds situated farther back. The anterior pair of these folds eventnally unite and form 

 the central or tubulär part of the siphon, and the more posterior folds form the lateral or valvulär 

 portions of the same organ. The rudimentary arms arise posterior to the mouth on the ventral 

 and lateral sides of the yolk-sac, and only Surround the buccal region at a later stage. The 

 yolk-sac occupies the same relative position, behind the mouth, as the central part of the foot- 

 area of ordinary Gastropod larvae in the early veliger stages. Therefore the arms are muscular, 

 lateral outgrowths of this same foot-area. The two lateral rows of rudimentary arms are widely 

 separated at first by the yolk, but during the absorption of this, they rapidly approach each other 

 and converge around the mouth." 



2 Owen, R., Memoir on the Pearly Nautilus. London, 1832, — Kerr, J. O., Anatomy of 

 Nautilus pompilius. Proc. Zool. Soc, London, 1895. — Griffin, L. E. , Anatomy of Nautilus 

 pompilius. Mem, Nat. Acad. Sei., 1900, vol. viii. 



