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. 256). 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, I1). 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 Fossia, 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 
Fhynchoteuthis. In some Oigopsida, such as Leachia, Chawnoteuthis, 
some species of Cheiroteuthis, and Grimalditeuthis (Fig. 258), the 
tentacular arms are reduced to mere stumps: in the adult Veranya 
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 7'remoctopus (Fig. 254), the six dorsal arms in /Zistioteuthis, 
and all eight arms in some species of Hledone, 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 
