190 INVERTEBRATE ANIMALS. 



able to transmit the sperm-cells to the female, but it remains 

 permanently attached to the animal. In the Paper Nautilus 

 (Argonaut) the process goes still further. The female of this 

 species (Fig. 90) attains a considerable 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 meta- 

 morphosed. 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 ; and 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 (Argonauta) and the 

 Pearly Nautilus (Nautilus 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. 90) is coiled into a spiral, but it is not di- 

 vided into chambers, and it is secreted by the webbed extrem- 

 ities of two of the dorsal arms of the female. These arms 

 are bent backward, so as to allow the animal to live in the 

 shell ; but there is no organic connection between the shell and 

 the body of the animal. The shell of the Pearly Nautilus, on 

 the other hand, is secreted by the mantle, and is organically 

 connected to the animal. It is coiled into a spiral (Fig. 91), 

 but it differs from the shell of the Argonaut in being divided 

 into a series of chambers by means of shelly partitions, which 

 are connected together by a tube or " siphuncle," the animal 

 itself living in the last and largest chamber only of the shell. 



The Cephalopoda are divided into two extremely dis- 

 tinct and natural orders, termed respectively Dibranchiata and 

 Tetrabranchiata, according as they have two or four gills or 

 branchiae. 



The Dibranchiata comprise the Cuttle-fishes, Squids, Cala- 



