SUB-CLASS i CEPHALOPODA TETRABRANCHIATA 



xnn-1'iinil flf imiiifh, and serve as prehensile and locomotive organs; in the 

 Dibranckiates //"'// area/rmed with //</> ////// *nd-t />.' 



The Cephalopoda are the most highly organised, and include some of the 

 largest of all the AMlusca. They breathe by gills, and are exclusively marine. 

 Their nervous, circulatory, digestive, and reproductive systems, their muscula- 

 ture, and sense organs all exhibit remarkable differentiation a.- 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. 



Recent Cephalopods were divided by Owen into two groups Tetralranchiata 

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

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

 of forms. A host of fossil Cephalopods abounded in the Palaeozoic and Meso- 

 zoic seas. The two largest groups of these, 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 hand, are more like those of certain Dibranchiates ; hence it is 

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

 classes. 



Sub-Class 1. TETRABRANCHIATA. Owen. 1 ' 



Cephalopods with four plum-like gills, and external, chambered shells. Amlniln- 

 t or u funnel divided ; ink-bag absent/ arms represented in exist in;/ XmiliU by lobes 



Fossils, vol. III., ibid. 1884-97. Contributions to Canadian Palaeontology, vol. I., 1885-89. De- 

 scriptions of Fossils from the Devonian of Manitoba (Trans. Roy. Soc. Canada, vol. VIII. sec. 4), 

 1890. Wh it field, R. P., Several papers in Bull. Amer. Mus. Nat. Hist,, especially I., No. 8, 1886 : 

 II., No. 2, 1889 ; III., No. 1, 1890 ; IX., No. 2, 1897. Republication of Hall's 'Fossils, etc. (//,/,/. 

 vol. I., Part II.), 1895. -Wright, T., Monograph on the Lias Ammonites (Palaeont. Soc.), 1878-86. 

 Wilrtenberger, R., Studien iiber die Stammgeschichte der Ammoniten (Darwinistische Schrifte, No. 5). 

 Leipzic, 1880. Zittel, K. A., Cephalopoden der Stramberger Schichten (Palaeont. Mittheil. Museum 

 Bayer. Staates, Bd. II.), 1868. Die Fauna der alteren Tithonbildungen (ibid. Bd. III.), 1870. 

 Handbuch der Palaeontologie, Bd. II., 1881-85. 



1 Professor A. E. Verrill, whose knowledge of existing Cephalopods is not excelled by that of 

 any other malacologist, has kindly furnished the following and some other notes at the request of 

 Professor Hyatt : 



"The arms, together with the siphon ( unbulatory funnel) of Ceph.alopods. must be considered 

 as homologous with the foot of other Mollujca. 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 (funnel) arise as two oblique pairs 

 of folds situated further back. The anterior pair of these folds eventually unite and form the 

 central or tubular part of the siphon, and the more posterior folds form tin- lateral or valvular 

 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 h'rst 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, 1 832. 



