} 
SUB-CLASS I CEPHALOPODA—TETRABRANCHIATA 505 
tacles surround the mouth, and serve as prehensile and locomotive organs; in the 
Dibranchiates they are armed with hooks and suckers. 
The Cephalopods are the most highly organised, and include some of the 
largest of all the Mollusca. 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 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. 
Recent Cephalopods were divided by Owen into two groups—Tetrabranchiata 
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, dmmonoidea and Bbelemnoidea, 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.” 
Cephalopods with four plumlike gills, and external, chambered shells. Ambula- 
tory funnel divided ; ink-bag absent ; arms represented in existing Nautili by [lags 
Fossils, vol. III., ébid. 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. see. 4), 
1890.— Whitfield, R. P., Several papers in Bull. Amer. Mus. Nat. Hist., especially I., No. 8, 1886 ; 
IL., No. 2, 1889 ; III., No. 1, 1890; IX., No. 2, 1897.—Republication of Hall’s Fossils, ete. (ibid. 
vol. I., Part II.), 1895.— Wright, T., Monograph on the Lias Ammonites (Palaeont. Soc.), 1878-86. 
Wirtenberger, 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 (7/7. 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 (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 (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 the 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 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. 

