CEPHALOPODA. 441 



Octopus; its base in Sepia, its whole length in Idiosepius and Eossia, and LoJiolus. 

 But in three genera, Philonexis, Trcmoctopus, and Argonauta, the modification 

 is much more extensive, and the arm affected is charged with spermatozoa, 

 cast off, and deposited in the mantle-cavity of the female (see above). This 

 is called an autotomous hectocotylus. The modification is as follows : before 

 sexual congress the arm in question is represented by a somewhat globular 

 sac, which consists of flaps of the basal part of the arm wrapped round the distal 

 part. Soon this sac bursts and allows the distal part and the suckers of the 

 basal part to appear (Fig. 355). The distal part thus set free has at its end 

 a, small sac, which in its turn bursts and allows a long terminal filament to 

 issue. The folds which formed the first-named sac, give rise to a receptacle on 

 the aboral side of the arm, which becomes charged with spermatophores. This 

 spermatophore receptacle communicates with a small vesicle in the base of the 

 arm, which leads into a long canal extending along the arm and filament, and 

 opening at the end of the latter. How this arm, which is deposited in the 

 mantle-cavity of the female, is used in fertilization, and how it becomes charged 

 with spermatophores is not known. After the arm is detached a new encysted 

 arm is said to be formed on the scar. 



Those Cephalopoda which are without this autotomous hectocotylus are said to 

 copulate mouth to mouth, the modified arm being used to affix the spermato- 

 phores to the buccal membrane, or to transfer them to the mantle-cavity of the 

 female. In Sepia and Loligo spermatophores are found in the pockets in the 

 buccal membrane, and in Nautilus within the annular lobe of the head (v. p. 421). 



Development. The egg of all Cephalopoda is large and heavily 

 charged with yolk. This is the case even with Nautilus, as may 

 be gathered from the ovarian egg, for the laid egg* of Nautilus 

 lias never been found. There is no larval stage, not even a trace 

 of the velum, and the young are hatched with the form of the 

 adult. The development is therefore very different from that of 

 other molluscs. 



In Dibranchs the cleavage is partial, and confined to one pole of the egg ; as 

 in the bird's egg it gives rise to a blastoderm or germinal disc, which in the 

 subsequent development is raised more and more from the subjacent yolk. Soon 

 several projections appear on the embryonic rudiment (Fig. 356). First in the 

 centre of the germ a flattened circular ridge is formed round a central depression ; 

 the ridge is the mantle, and the depression is the shell-gland. In Octopoda 

 (Argonauta) the shell -gland seems to shallow 7 out and disappear, but in 

 Decapods it becomes closed over and persists as a sac in which the shell is 

 deposited. 



On each side of the mantle the two parts of the funnel (Tr), which even in 

 Dibranchs are separate in the embryo, appear ; and between these and the 

 mantle the gills (Br). Also laterally, but external to the folds of the funnel, 

 the first traces of the head appear as two pairs of elongated lobes, of which the 



* Since this was written, the eggs of Nautilus have been found by Dr. Arthur 

 Willey, the Balfour student of the University of Cambridge (Proc. Roy. Soc., 

 vol. 60, 1897, p. 467). The eggs, which are laid singly, are enclosed in a double 

 capsule of cartilaginous consistency. The ovum is very large (17 mm. in its 

 longest diameter) and is surrounded by albumen. 



