THE CEPHALOPODA 289 



forming the so-called "organ of Valenciennes," peculiar to the 

 female and serving for the attachment of the spermatophores 

 (Fig. 255, m). 



In the Dibranchia the pedal appendages have the form of four 

 or five pairs of symmetrical and generally elongate arms. In the 

 Octopoda there are eight similar arms, and the whole length of the 

 ventral surface of each is covered by suckers which are often very 

 numerous and highly specialised in structure (Fig. 25C). It seems 

 probable that it is the suckers and not the arms that are com- 

 parable with the tentacles and tentacle -sheaths of Nautilus. In 

 the Decapoda, in addition to the eight arms corresponding to those 

 of the Octopoda, there are two additional "tentacular" arms, of 

 which one is situated between the third and fourth sessile arms on 

 either side of the posterior part of the head. These two tentacular 

 arms are longer and more slender than the others (Fig. 298, A), and 

 the suckers are generally confined to their free extremities, which 

 are enlarged and club-shaped ; in some forms, however, they bear 

 suckers along their whole length (Fig. 297, II). The tentacular arms 

 are further distinguished from the sessile arms by the fact that they 

 are more or less retractile within special pouches : they are com- 

 pletely retractile in Sepia, Sepiola, and Rossia, incompletely retractile 

 in Loligo, very slightly retractile in the majority of the Oigopsida, 

 and finally they are united to form a beak -like appendage in 

 Rhynchoteuthis. In some Oigopsida, such as Leachia, Chaunoteuthis, 

 some species of Cheiroteuthis, and G-rimalditeuthis (Fig. 258), the 

 tentacular arms are reduced to mere stumps : in the adult Vemnya 

 they are similarly reduced, but the young still retain small 

 tentacular arms. Similarly a notable reduction of the ordinary or 

 sessile arms, particularly on the dorsal side, may be observed in 

 some Cheiroteuthidae and Cranchiidae. Some or all of the eight 

 sessile arms may be united by a more or less complete inter- 

 brachial membrane : the four dorsal arms are united in this 

 manner in Tremodopus (Fig. 254), the six dorsal arms in Histioteuthis, 

 and all eight arms in some species of Eledone, in Alloposus, and in 

 the adult Cirrhoteuthidae and Amphitretidae, the membrane ex- 

 tending in the two last-named families to the tips of the arms, 

 but in the young of Cirrhoteuthis (Fig. 260) the membrane is not 

 fully developed. 



In the female Argonauta the two dorsal arms are enlarged to 

 form a veil (Fig. 301, IV), which is applied to the mantle and 

 secretes a protective calcareous shell. Finally, in most cases a 

 single arm of the male, or a portion of the circumoral pedal crown 

 in Nautilus, is modified to form a copulatory organ, which is some- 

 times detachable. This is the hectocotylus, or spadix in Nautilus, 

 which will be described in detail under the head of reproductive 

 apparatus. 



19 



