23 



from the cup by the inpushed base and "piston", while when the arm is 

 withdrawn , they hold by suction , often , until the pedicle breaks. 



Argonauta and other Octopoda have one of the arms transformed into a 

 sac, which, after receiving the spermatophores , is detached and placed in the 

 mantle cavity of the female. This modified arm is called the Hectocotylus. The 

 Decapoda have one or more rarely two of their arms slightly modified or 

 "hectocotylized". In Loligo (Plate III, Fig. 18) the left fifth arm is hectocotylized. 

 Some ten or twelve suckers on each side of the tip of the arm have their pedicles 

 enlarged and their cups reduced , the pedicles becoming large blunt cones bearing 

 minute cups or near the end of the arm both stalk and sucker are reduced. 

 The inner row of suckers is less modified than the outer. The portion of the 

 arm between the two rows of enlarged pedicles forms a rounded ridge as 

 high as the pedicle. The hectocotylus probably represents the vestige of a 

 functional structure of an ancestral form. It is barely 

 possible that the arm may be used for the transference of 

 spermatophores. 



The siphon or funnel (text figure 10) developes from 

 two pairs of ridges which appear upon the posterior surface 

 of the embryo. The hinder pair appears between the mantle 

 and the eye , and forms the siphonal valves which will be 

 described later. The anterior pair lies below the gills and 

 anus and between them and the arms. The latter ridges 

 become elevated and incurved until their edges meet and 

 unite , thus forming a conical tube , the siphon , which 

 leads from the mantle cavity to the exterior. This muscular 

 tube is attached dorsally by a pair of large muscles, the 

 siphonal retractors. Each siphonal retractor is a strong 

 rounded muscle which, arising from the side of the ventral 

 end of the vane of the pen , passes obliquely downward 

 and forward in a depression between the liver above and in front, and the 

 stomach , caecum , and nephridia below and behind , and terminates in one side 

 of the siphon. Viewed from below, the two siphonal retractors appear as 

 parallel bands which are separated at the back by the anterior vena cava and 

 which disappear between the nephridia and the gills. 



The arrangement of the intrinsic muscles of the siphon is very similar so 

 that of the mantle fibres except that there are longitudinal fibres which are 

 apparently continuous with those of the retractor. The majority of these fibres 

 form two pairs of bundles ; an upper pair which lies near the median line , and 



Fig. 10. 



