V THE HECTOCOTYLUS ARM IN CEPHALOPODA 1 37 



large nidamental glands^ which open into the mantle cavity inde- 

 pendently of the oviduct. Their purpose is to produce a viscid 

 mucus, which envelops the ova at the moment of their emission 

 and eventually hardens into the egg-capsules. A pair of acces- 

 sory nidamental glands occur in Sepia^ as well as a pair of 

 smaller glands situated on the oviduct itself. 



In many of the male Cephalopoda the vas deferens is long 

 and dilated at its outer end into a glandular reservoir, within 

 which are formed the spermatophores, or narrow cylindrical 

 packets which contain a very large number of spermatozoa. 

 When charged, the spermatophores pass into what is known as 

 Needham's sac, where they remain until required for use. These 

 spermatophores are a very characteristic part of the reproductive 

 arrangements in the Cephalopoda. The male of Sepia has been 

 noticed to deposit them, during union, upon the buccal mem- 

 brane of the female. During the emission of the ova by the 

 female, the spermatophores, apparently through the agency of 

 a kind of spring contained at one end, burst, and scatter the 

 spermatozoa over the ova. 



The Hectocotylus Arm. — Perhaps the most remarkable 

 feature in the sexual relations of all the Mollusca is the so-called 

 hectocotylus of the Cephalopoda. In the great majority of the 

 male Cephalopoda, one of the ' arms,' which is modified for the 

 purpose in various ways and to a greater or less extent, becomes 

 charged with spermatophores, and sometimes, during union, 

 becomes detached and remains within the mantle of the female, 

 preserving for some considerable time its power of movement. 



The hectocotylus is confined to the dibranchiate Cephalopoda, 

 and its typical form, i.e. when part of the arm becomes disen- 

 gaged and left with the female, occurs only in three genera 

 of the Octopodidae, viz. Argoiiauta, Ocythoe (^PJiilonexis)^ and 

 Tremoetopus. In all of these, the male is many sizes smaller 

 than the female. In Argonauta the third arm on the left side 

 becomes hectocotylised. At first it is entirely enveloped in a 

 kind of cyst, in such a way that only a small portion of the tip 

 projects ; subsequentl}^ the cyst parts asunder, and allows the 

 arm to become expanded to its full length, which considerably 

 exceeds that of the other arms. At a certain point the acetabula 

 or suckers terminate, and the remainder of the arm consists of 

 a very long, tapering, sometimes thread-like filament, which is 



