324 Introduction to Animal Morphology. 



sometimes a long surface groove communicating 

 therewith. Sometimes one arm, with few suckers 

 (Sepia), or with a spoon-shaped end (Octopus, Eledone),. 

 is set apart as a male sex-organ, and reaches a large 

 size, having a filamentary extremity and a Need- 

 hamian vesicle at its free end ; this, being set free, 

 enters the mantle cavity of the female, and thus im- 

 pregnates it. This arm is called a Hedocotylus ; it is 

 the third right arm in Octopus Carena, and Tremoc- 

 topis, the third left in Argonauta, and was formerly 

 regarded as a trematode parasite. A new arm is 

 produced when the hectocotylus is detached. In 

 Nautilus the oviduct opens unsymmetrically on the 

 right. The spermatozoa are united by the secretion 

 of the caecal pouch into cylindrical spermaphores, the 

 moving filaments oi Nccdhavi. 



The ova undergo only very partial segmentation 

 at a germinal flat disc at the pointed end of the ovum ; 

 here a cap of cells becomes visible {klastoplasts\ and in 

 the underlying substance a series of bodies form, 

 which look like nuclei, but without differentiated cell 

 areas round them [autoplasts) ; these become branching 

 contractile cells, in which inheres the rhythmically 

 contractile power of the yelk sac. 



After segmentation, the mantle, gills, and epipodia 

 form, then the sense-organs, then the foot and its mar- 

 ginal processes.* There is no metamorphosis, and the 

 organs are formed by the time the shell breaks. 



Of the 2000 known species, only 218 are now living. 

 They are divided into two orders : — 



I. Tetrabranchiata {Owen) — shell siphonate, camerated ; 



* Most of our knowledge of the embryogeny of Cephalopods we owe 

 to Mi: Lankester'?, researches, and to those of Salensky. 



