DIBRANCHIATA. 



109 



Decapods called Sepiaclce. The common calamary (Loligo), in 

 which the internal shell is reduced to a horny quill-shaped 

 plate, represents the fourth and most extensive family of the 

 present tribe called Teuthidce; and in which one genus 

 (Enoploteuthis) had the caruncle of its acetabula produced 

 into horny claws. In all the Decapods the mantle supports a 

 pair of fins, and the siphon is generally provided with a 

 valve. 



Fig. 33. 

 The Argonaut. 



In the tribe Octopoda, fins are rarely developed from the 

 mantle; but the eight ordinary arms are longer, thicker, and 

 are united together by a broader web, which forms a powerful 

 organ for swimming in a retrograde direction. One family in 

 this tribe (Testacea) is represented by the genus Argonauta 

 (fig. 33), in which, in the female sex, the first or dorsal pair of 

 arms, i, is dilated at its extremity into a broad thin membrane, 

 like the mantle in the testaceous Mollusks; by means of 

 these membranes the animal, in fact, forms for itself an 



