274 MANUAL OF ZOOLOGY. 



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 de- 

 scribed as a parasitic worm under the name of ' Hectocotylus ;' 

 subsequently 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, sometimes 

 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 * splanchnoskele- 

 ton.' In Spirula the phragmacone is the sole internal skele- 

 ton, 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 (Argonauta), and the 

 Pearly Nautilus (Nautilus pompilius) ; 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 

 is involuted, but is not divided into chambers, and it is se- 

 creted 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 re- 

 garded as a 'pedal shell.' The shell of the Pearly Nautilus, 

 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 ot 

 two branchiae only, and comprises the Cuttle-fishes, Squids, 

 and the Paper Nautilus. The latter is distinguished by the 





