CEPHALOPODA. 



67 



especially Crustacea. Some of them attain a great size. The flesh is 



eaten, and the colouring matter 

 of the ink-sac (sepia) and the 



'&L/ ^C^ ^^v \ dorsal shell (os sepice) are used 



by man. The remains of Cepha- 

 lopods occur in all formations 

 from the oldest Silurian and 

 constitute important charac- 

 teristic fossils (Belemnites, Am- 

 0c\ ~ / monites). 



Order 1 . TETRABRANCHI ATA. * 



Cephalopoda faith four gills 

 in the mantle 

 cavity and nu- 

 merous retractile 

 tentacles on the 

 head, with split 

 funnel and 

 many- chambered 

 shell 



The append- 

 ages of the head 

 are peculiar. In 

 place of the arms 

 there are a num- 

 ber of filiform 

 tentacles round 

 the mouth. In 

 Nautilus there are on each side 

 of the body (a) nineteen ex- 

 ternal tentacles, of which the 

 dorsal pair constitutes a kind of 

 hood which can close the orifice 

 of the shell; (b) two ocular 

 tentacles on each side near the 



FIG. 541. Almost 

 ripe embryo of 

 Sepia ojficinalis 

 from the dorsal (an- 

 terior) face (after 

 Kolliker). Ds, 

 yolk sac. 



FIG. 540. Embryonic development of Sepia offici. 

 nalis (after Kolliker). a, View of germinal 

 disc from above, commencing embryo lying on 

 the yolk. Er, gills ; Tr, folds of the funnel ; 

 Oc, eye ; M, mantle. 6, Somewhat' older stage, 

 seen from the front. D, yolk ; KV, anterior ; 

 Kl", posterior cephalic lobe ; 0, mouth, c, later 

 stage from the side. 1-4, first rudiments of 

 the arms, d, older stage from the front. 5, 

 fifth pair of arms, e, Still later stage in lateral 

 view The halves of the funnel have united. 



* Van der Hoeven, " Beitrage znr 

 Kenntniss von Nautilus " (in Dutch), 

 Amsterdam, 1856. 



W. Keferstein in Bronn, Classen 

 und Ordnungen des Thierreicbs. . 

 Critter Band, Cephalopoda. 1865. 



