TYPE M0LLU8CA. 357 



spermatophore. The terminal portion of the arm, which is traversed 

 throughout its entire length by a canal, is developed, into a long terminal 

 filament through which the spermatozoa may pass. During copulation 

 the arm is probably thrown off and passes into the mantle-cavity of the 

 female, the manner in which the spermatozoa reach the ova being, however, 

 not yet understood. When first discovered in the mantle-cavity of a female 

 the arm was regarded as a parasitic worm, and the name Hectocotylus was 

 applied to it — a term which is still retained on account of its convenience. 

 In other genera of Cephalopods one arm is generally peculiarly modified in 

 the male — in the Decapoda usually the fourth of the left side and in the 

 Octopoda usually the third of the right side, though frequent exceptions are 

 found. This arm is termed the hectocotylized arm, though it is doubtful 

 whether it takes any part in copulation. 



As will be seen from the above description the genus 

 Nautiliis differs in many important particulars from the re- 

 maining genera of Cephalopods, and the class is therefore 

 divided into two orders. 



1. Order TetrabrancMa. 



This order, of which the genus Nautilus (Kg. 159) is the 

 sole living representative, was in former periods of the earth's 

 history the dominant group of the Cephalopods — the Ortho- 

 cerites of the Palaeozoic and the Ammonites of the Mesozoio 

 being extinct members of it. It is characterized by its mem- 

 bers possessing four ctenidia, four auricles to the heart, and four 

 nephridia; and in addition there may be mentioned, as further 

 peculiarities, the presence of paired reproductive ducts, of 

 which the right one alone is functional, and also of direct 

 communication of the viscero-pericardial cavity with the ex- 

 terior by two pores, and by the occurrence of a single pair of 

 osphradia. For a more detailed account of the peculiarities 

 of Nautilus the preceding general description may be con- 

 sulted. It remains to discuss here the shell and the structure 

 of the foot-lobes — structures which, with the other characters 

 mentioned, serve to distinguish Nautilus from all its living 

 congeners. 



The shell is voluminous, coiled, and calcareous, its cavity 

 being divided by a series of transverse partitions into a num- 

 ber of chambers, in the last — that is to say, the youngest — of 

 which tlie animal lives, while the remaining ones are filled with 



