THE PAST AND PRESENT OF THE CUTTLEFISHES, 103 



these relationships and of the ways of development within our grasp. 

 Thus a study of the cuttlefish eye proves that, whatever its com- 

 plexities, it represents the advanced and modified result of the de- 

 velopment of lower molluscan eyes. Such a study also corrects 

 erroneous notions of the genealogy of the animal world. The pre- 

 sumed relationship between vertebrate and cuttlefish eyes disappears 

 at once under the light of Hensen's researches. As Darwin so well 

 puts it, in speaking of the difference between these two eyes, " The 

 crystalline lens in the higher cuttlefish (fig. 7, B) consists of two 

 parts (L 1 , L 2 ), placed one behind the other like two lenses, both 

 having a very different structure and disposition from what occurs in 

 the Vertebrata. The retina is wholly different, with an actual inver- 

 sion of the elemental parts, and with a large nervous ganglion in- 

 cluded within the membranes of the eye." Then, in further detailing 

 the disappearance of the difficulties started by Mr. Mivart, Darwin 

 says, " It is of course open to any one to deny that the eye in either 

 case (cephalopods or vertebrates) could have been developed through 

 the action of natural selection of successive slight variations ; but 

 if this be admitted in the one case, it is clearly possible in the other ; 

 and fundamental differences of structure in the visual organs of two 

 groups might have been anticipated, in accordance with this view of 

 their manner of formation. As two men have sometimes indepen- 

 dently hit on the same invention, so in the several foregoing cases it 

 appears that natural selection, working for the good of each being, 

 and taking advantage of all favourable variations, has produced 

 similar organs, as far as function is concerned, in distinct organic 

 beings, which owe none of their structure in common to inheritance 

 from a common progenitor." 



Passing now to consider the reproduction and development of 

 the cuttlefishes, we note that the two sexes are completely defined, 

 and that the young are developed, as are those of all other animals, 

 'from eggs. Certain curious details are connected with the act of 

 egg-fertilisation in this group. The males exhibit an unsymmetrical 

 condition of their arms in that one or more of these organs becomes 

 specially developed to form what is known as the hectocotylus^ and by 

 which fertilisation of the ova is effected. This arm is the third right 

 arm in octopus, and the third left in the argonaut (fig. 9, c, a). When 

 first discovered, this hectocotylised arm was regarded as a parasitic 

 organism. In sepia it is the base of the arm which undergoes altera- 

 tion, whilst in octopus it is the tip which is most modified, the extremity 

 of the arm being converted into a spoon-like process. In the argo- 

 nauts, and in Tremoctopus as well, the highest modification of the arm 

 for reproduction is witnessed. Here, the arm itself is formed within 

 a sac or vesicle, from which, at maturity, it is released. Its extremity 

 is prolonged to form a whip-like lash, and this modified organ is itself 



