104 The Origin and Transformation of Animals. 



developing themselves on the outside, as with the polyps and 

 the medusas. When the temperature falls, the normal repro- 

 ductive apparatus shows itself in distinct individuals, and then 

 we find males and females, that is to say, true proglottis " M. 

 Quatrefages does not consider that the main facts demand any 

 other interpretation, in consequence of Heiden's discovery that 

 an aphis, after producing offspring all through the season 

 agamically, or in a spinster state, ends in acquiring sexual 

 characteristics. This he calls the scolex becoming a proglottis.* 



With reference to the Salpa and its curious " alternation of 

 generation," M. Quatrefages observes : " Thanks to Messrs. 

 Xrohn and Huxley, we now know that with the Salpse there is 

 not only an alternation in form and condition, but also in the 

 method of reproduction. From their united labours, it appears 

 that the chained Salpre are at once males and females, and that 

 they lay eggs from which the isolated Salpee are produced. 

 These last are neuters, and give rise, by internal budding, to 

 chained Salpse only. . . . Among the Salpse it is as if the 

 egg of a butterfly produced a caterpillar, from which sprung a 

 chain of butterflies fastened together, and flying without power 

 of separation." 



The phenomena of geneagenesis are confined to the lower 

 grades of the animated world : no vertebrate animal exhibits 

 them, and they are rare amongst invertebrates of an elevated 

 organization. Independent of the plant-lice, insects rarely fur- 

 nish instances of this peculiar mode of multiplication, and M. 

 Quatrefages tells us that among the superior annulated animals, 

 or those possessing articulated feet, it is only found among in- 

 sects and crustaceans. " Moreover, among these latter we have 

 no other example than that afforded by the Daphina ; at least 

 nothing of the kind has yet been noticed in the myriopods, 

 spiders, and cirrhipedes.f 



Among the worms, or inferior annulated animals, it is com- 

 monly found, and M. Quatrefages regards flssiparity or multi- 

 plication by division as belonging to this system of reproduc- 

 tion. He observes, with special reference to Nais, Nemertes, and 

 other worms, " During many generations the individuals pro- 

 duced by this method are neuters like their parent ; at length, 

 under conditions which are not known, the sexes appear, and 

 the species is propagated afresh by means of eggs." He adds, 

 that no mollusc, properly so called, adopts this mode of propa- 

 gation, but that among the molluscoida (or mollusc-like crea- 

 tures) geneagenesis seems to be the rule. 



* Detailed information concerning this subject will be found in Mr. Huxley's pa- 

 per on the Organic Reproduction and Morphology of the Aphis. Linn. Trans. 1858. 



f The Scolopendra, or " Hundred Legs," common in gardens, is a myriopod ; 

 the acorn barnacle, so frequent on seaside rocks, is a cirrhipede. 



