CUTTLE-FISH AND TAPER-NAUTILUS. 



647 



alnrays longer than the body, and the suckera of which have no horny rings. 



In many the arms are connected by a web, the umhrdla, which may exist 



only at their base or extend almost to their tips, as in Cirroteuthin; it is 



probably of assistance in swimming. 



There la no shell except in the famous " Paper-Nautilus " (Argonauta), and 



then it is not a true shell but a secondary one. This is carried by the 



females only, and is formed after the 

 animal is hatched. It consists of a 

 thin, white, perfectly open shell, which 

 is slightly coiled towards the back ; its 

 corrugated walls are formed of three 

 layers — the outer ones prismatic, the 

 inner fibrous. The animal nestles in 

 the boat-like shell, which is in nowise 

 attached to the body, and holds it in 

 place with the two front arms. The 

 ends of these arms are curved round 

 and sustain thin membranes, vela, 

 which cover the exterior of the shell 



on either side (Fig. 25). Whether the mantle takes part in the secretion of 



this shell, or whether it is wholly formed by the "vela," is not yet quite 



clear ; its function, however, is probably less to protect the animal than to 



serve as a receptacle for the eggs.. 



The following families are included in the Octopoda: — Cirroteuthidse, 



Amphitretidse, Argonautidai, Tremoctopodidije, AUopsidai, and Octopodidas. 



a. 25. — Arqonadt. 



