242 



COMPARATIVE ANATOMY 



CHAP. 



c. The copulatory apparatus Hectocotylisation in the Cephalopoda. The 



copulatory apparatus of the Gastropoda, and the penis which projects into the 

 mantle cavity in certain Cephalopoda have already been described. 



One of the most remarkable and enigmatical phenomena in connection with 

 the Cephalopoda is their hectocotylisation. This consists in the transformation of 

 one of the oral arms of the male into a copulatory organ and 

 spermatophore-carrier. This arm is said to be hectocotylised ; 

 during copulation it becomes detached, and finds its way into 

 the mantle cavity of the female. 



Typical hectocotylisation (Fig. 200) is found only in the 

 Oetopodan genera Argonauta, Philonexis, and Trcmodopus. In 

 Tremoetopus and Philoncxis (Parasira) the third arm on the 

 right is the one transformed, in Argonauta the third on the left. 

 The arm is at first enclosed in an outwardly pigmented sac (Fig. 

 200 A), when this bursts, the arm becomes free, and then its 

 special form can be recognised (B). The folds which formed 

 the sac bend back so as to form a new sac, which receives the 

 spermatophores and is now inwardly pigmented. An aperture 

 leads from this sac into a seminal vesicle inside the hectocotylised 

 arm ; this vesicle is continued into a long thin efferent duct, 

 which runs the whole length of the arm and opens outwardly 

 at its end. The end of the arm is transformed into a long 

 filamentous penis, which at first is also enclosed in a special sac. 

 When the penis is evaginated the sac remains as an appendage 

 at its base. 



The spermatophores then pass from the pigmented sac into 

 the seminal vesicle, and are ejected through the efferent duct 

 which opens at the tip of the penis. 



It is probable that Cephalopods grasp one another, during 

 copulation, with their arms, in such a way that their mouths face 

 each other. In this position the hectocotylised arm of the male 

 becomes detached, and in some way or other forces its way 

 into the mantle cavity of the female. Detached arms are often 

 found in the mantle cavity of the female, as many as four have 

 been found at one time. 



We still do not know (1) how the hectocotylised arm fertilises 

 the eggs of the female, or (2) how the spermatophores reach the 

 hectocotylised arm. 



The males and females in the above-mentioned genera differ 

 from one another, apart from the sexual dimorphism caused by 

 the development of the hectocotylised arm. The males are 

 much smaller, and in Argonauta the female only has a shell. 



It is very probable that the detached hectocotylised arm 

 can be replaced by a new one. 



Although a true hectocotylised arm, which can be detached, 

 is only developed in the three genera above mentioned, it has 

 been proved that in all other Cephalopoda (even Nautilus, cf. p. 117), a certain arm 

 or portion of the head in the male is in some way modified, differing in some 

 (often unimportant) manner from the other arms. Such an arm is said to be 

 hectocotylised, and it is assumed that it plays some part in copulation, although 

 its exact function is unknown. In Sepia and Nautilus it is even difficult to 

 imagine what part it can take in copulation. The constant occurrence of 



remarkable as 



Fio. 199. Spermato- 

 phore of Sepia (after 

 Milne Edwards), c, 

 Outer case ; 6, inner 

 case ; c, spermatozoal 

 sac ; d, e, f, g, various 

 parts of the ejacula- 

 tory apparatus. 



hectocotylised arm is the more 



it is by no means always 



a 

 the 



