330 MOLLUSCS. 



Class Cephalopoda. Cuttlefishes. 



This class includes Squid and Calamary, Octopus and 

 Sepia, the Argonaut and the pearly Nautilus, besides hundreds 

 of extinct forms — such as Belemnites and Ammonites. 



Characteristics. — Cephalopods swim in the sea, or lurk 

 and creep among the rocks, voracious devourers of many 

 kinds of prey — hard crabs and blubbery jellyfish. 



They cannot be mistaken for any other animals, for their 

 forms and their ways are alike peculiar. 



Like most free-swimming molluscs, they are bilaterally 

 symmetrical, but part of the " foot " has come to surround 

 the head, and is divided into numerous "arms." Another 

 (middle) part of the foot forms a partially or completely 

 closed tube — the " siphon " or " funnel " — through which 

 water gushes out from the mantle-cavity, driving the animal 

 backward. The muscular mantle-flap sheltering the gills is 

 posterior in position, and the visceral mass, though without 

 any spiral coiling, is much elongated in a direction which is 

 anatomically dorsal and posterior, but which points forwards 

 when the cuttlefishes propel themselves through the water. 



A chambered external shell serving as a house is present 

 in Nautilus alone among living Cephalopods. In Spirula, 

 there is a spiral, chambered shell, but it is covered by the 

 mantle, and is too small to serve as a house. Most of the 

 extinct forms lived in shells, tubular, or curved like a horn, or 

 chambered Wks tYiSX oi Nautilus ; most of the modern forms 

 s6em to be more active than their ancestors, and their shells 

 have degenerated. I shall summarise what Ray Lankester 

 says as to the degeneration. It seems that the shell was re- 

 duced in size, became surrounded by the mantle in a secondary 

 shell-sac, that the walls of this sac secreted lime which dis- 

 guised the enclosed shell (e.g., Belemnites), that the original 

 shell disappeared while the secondary deposits of lime were 

 left (Sepia), that even this secondary calcareous deposit may 

 disappear, leaving an organic chitinous plate (e.g., the pen 

 of Loligo), and that even this may disappear, e.g., in Octopus. 

 One must be careful to note that the " cuttle-bone " of Sepia 

 is not a rudiment of an old shell, and that the secondary 

 shell-sac is different from the embryonic shell-gland. 



