CEPHALOPODA. 209 
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 different 
species of Cuttle-fish. Inthe species figured opposite (Octopus 
carena, fig. 111), the third right arm is at first developed in a little 
bladder or cyst. After a time the cyst ruptures, when the meta- 
morphosed arm is liberated, and then appears to be of much 
greater size than the corresponding arm on the left side, and to 
terminate in an oval plate (fig. 111, 2). This plate appears to 
have the function of transmitting the sperm-cells to the female, 
but the arm itself remains permanently attached to the animal. 
In the Paper Nautilus (Argonaut) the process goes still 
further. The female of this species (fig. 112) attains a consider- 
able size, and is protected by an external shell. The male 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 the purpose of conveying the sperm-cells to 
the female. 
The shell of the Cephalopoda is sometimes external, some- 
times internal. The internal skeleton is seen in the various 
Cuttle-fishes, in which it is known as the “cuttle-bone” or 
“pen.” It may be-either horny or calcareous, and it is some- 
times complicated by the addition of a chambered portion. 
The only living Cephalopods which are provided with an ex- 
ternal shell are the Paper Nautilus (4~gonauta) and the Pearly 
Nautilus (Wautilus pompilius) ; but not only is the structure of 
the animal different in each of these, but the nature of the shell 
itself is entirely different. The shell of the Argonaut (fig. 112) 
is coiled into a spiral, but it is not divided into chambers, and 
it is secreted by the webbed extremities of two of the dorsal 
O 
