MOLLUSCA I CEPHALOPODA. 349 



small worm-like body, with a filiform appendage in front, with 

 two rows of alternating suckers, and a dorsal sac with nume- 

 rous " chromatophores." The duct of the testis probably opens 

 into the base of the hectocotylus, which is ultimately detached, 

 and is deposited by the male within the pallial chamber of the 

 female. When first discovered in this position, it was described 

 as a parasitic worm under the name of " Hectocotylus ; " sub- 

 sequently it was described as the entire male, and it is only 

 recently that its true nature has been fully ascertained. 



The shell of the Cephalopoda is sometimes external, some- 

 times internal. The internal skeleton is known as the " cuttle- 

 bone," " sepiostaire," or "pen" (gladius), and may be either 

 corneous or calcareous. In some cases it is rendered complex 

 by the addition of a chambered portion or " phragmacone," 

 which is to be regarded as a visceral skeleton or " splanchno- 

 skeleton." In Spirula the phragmacone is the sole internal 

 skeleton, and is coiled into a spiral, the coils of which lie in 

 one plane, and are near one another, but not in contact. It 

 thus resembles the shell of the Pearly Nautilus, but it is internal, 

 and differs, therefore, entirely from the external shell of the 

 latter. The only living Cephalopods which are provided with 

 an external shell are the Paper Nautilus (Argonautd), and the 

 Pearly Nautilus (Nautilus pompilius) j but not only is the struc- 

 ture of the animal different in each of these, but the nature of 

 the shell itself is entirely different. The shell of the Argonaut 

 (fig. 146) is involuted, but is not divided into chambers, and 

 it is secreted by the webbed extremities of two of the dorsal 

 arms of the female. The arms are bent backwards, so as to 

 allow the animal to live in the shell, but there is in reality no 

 organic connection between the shell and the body of the 

 animal. In fact, the shell of the Argonaut, being confined to 

 the female, and serving by its empty apex as a receptacle for 

 the ova, may be looked upon as a " nidamental shell," or as 

 it is secreted by a modified portion of the foot, it may more 

 properly be regarded as a " pedal shell." The shell of the 

 Pearly Nautilus (fig. 148), on the other hand, is a true pallial 

 shell, and is secreted by the body of the animal, to which it is 

 organically connected. It is involuted, but it differs from the 

 shell of the Argonaut in being divided into a series of chambers 

 by shelly partitions or septa, which are pierced by a tube or 

 " siphuncle," the animal itself living in the last chamber only 

 of the shell. 



The Cephalopoda are divided into two extremely distinct and 

 well-marked orders, termed the Dibranchiata and the Tetra- 

 branchiata. The former is characterised by the possession of 



