CEPHALOPODA. 



211 



structure, therefore, is a decided approach to the Vertebrate type of 

 organisation. 



The sexes in all the Cephalopoda are in different individuals, and 

 the reproductive process in the Cuttle-fishes is attended with some 

 singular phenomena. The most remarkable point in this connection 

 is the modification of one of the arms of the male Cuttle-fishes, for 

 the purpose of conveying the male element to the female. The 

 details of the modification vary in difi'erent species of Cuttle-fish. 



In some of the Cuttle-fishes the modified arm becomes the recep- 

 tacle of the spermatozoa, but is not itself actually detached from the 

 animal. In some cases, however, as in the Paper Nautilus (Argo- 

 nauta), the process goes still further. The female of this species (fig. 

 151), attains a considerable size, and is protected by an external shell. 



Fig. 150.— a Male of Argonauta argo, witli the modified reproductive arm still con- 

 tained in its enveloping cyst, four times enlarged (after H. MuUer) ; b Detached 

 reproductive arm or hectocotylus of Tremoctopus viobceun (after KoUiker). 



The male (fig. 150, a) is not more than an inch in length, is devoid 

 of a shell, and has its third left arm metamorphosed. This arm is 

 developed in a cyst, and is ultimately detached from the body, and 

 deposited by the male within the mantle-cavity of the female. When 

 first discovered in this position, it was described as a worm living 

 parasitically on the Argonaut, under the name of " Hectocotylus " 

 (Gr. hekaton, a hundred ; kotulos, a cup), from the suckers or cups 

 with which it was furnished. Subsequently it was described as the 

 entire male Argonaut ; and it is only recently that it has been proved 

 to be nothing more than one of the arms of the male, detached for 



