104 STUDIES IN LIFE AND SENSE. 



ultimately destined to be detached from the head of its possessor, 

 and to be directly applied to the fertilisation of the ova. 



The development of the cuttlefishes presents us with a subject of 

 exceeding interest. That of nautilus is unknown ; but the develop- 

 ment of the two-gilled forms shows us exactly what the theory of 

 their evolution would lead us to expect namely, that, as representing 

 an extremely ancient and modified molluscan stock, the phases of 

 their development should be more or less obscured, concentrated, 

 and modified. A long ancestry leaves its mark on the development 

 of a class. The modifications of form, the shifting environments, 

 and the other conditions of organic change, to which an ancient race 

 must unquestionably have been subjected, undeniably tend to shorten 

 the developmental process, and to obliterate the ancestral phases so 

 plainly seen, as a rule, in life-histories of shorter extent. The egg 

 undergoes partial segmentation, and the blastoderm or germinal mem- 

 brane, from the substance of which the embryo is formed, beginning 

 to appear on one aspect of the egg, soon extends over the whole 

 egg-surface. This blastoderm soon develops its three characteristic 

 layers, hypoblast, mesoblast, and epiblast. Then, in the middle of 

 the blastoderm, appears a patch of substance representing the future 

 mantle or investing skin of the body. In the centre of this mantle 

 is a depression which gives origin to the shell, and is named the 

 " shell-gland." Two curved folds on each side become the " funnel." 

 In front the two eyes are developed. Behind the mantle are two 

 buds which ultimately develop into the gills. The arms likewise 

 begin to appear at an early stage of development, whilst the head- 

 fold, with which the arms are to be so intimately related, is developed 

 as two swellings, one on each side of the growing mass. Even after 

 the embryo has attained a tolerable size, the " yolk-sac "persists, 

 its substance being gradually absorbed to afford the wherewithal for 

 the building of the new and developing frame. On comparing the 

 development of a cuttlefish with that of an ordinary mollusc, such as 

 the whelk or mussel, grave differences in development, alluded to in 

 a former part of this discourse, are apparent. Thus the embryo 

 cuttlefish is exceptional in possessing an external yolk-sac ; it shows 

 no " foot " in the course of its development ; instead thereof it 

 develops the characteristic arms ; and it lastly has no " velum " or 

 ciliated disc, which forms such a prominent feature in the develop- 

 ment of other Mollusca. Cuttlefish development, exhibiting in its 

 phases a concentration of some and an obliteration of other molluscan 

 characters, testifies to the modification, as well as to the progressive 

 development, which the race in time has undergone. 



The present history of the cuttlefishes may be concluded by the 

 briefest possible reference to their distribution and classification. 

 Over 2.000 species of cephalopods are known. But geology claims 



