240 ORIGIN AND TRANSFORMATION OF ANIMALS. 
Other interpretation, in consequence of Heiden’s discovery 
that an Aphis, after producing offspring all through the season 
afjamically or in a spinster state, ends in acquiring sexual 
characteristics. This he calls the scolex becoming a pro- 
gloXtis.* 
With reference to the Salpa and its curious “alternation 
of generation,^^ M. Quatrefages observes—“ Thanks to Messrs. 
Krohn and Huxley, we now know that with the Salpae 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 Salpae are at once males and females, 
and that they lay eggs from which the isolated Salpae are 
produced. These last are neuters, and give rise, by internal 
budding, to chained Salpae only. . . . Among the Salpae 
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 
furnish 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 insects 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 fissijoarity, or mul¬ 
tiplication by division, as belonging to this system of repro¬ 
duction. He observes, with special reference to Nais, 
Nemertes, and other worms, “ During many generations, the 
individuals produced 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 propagation, but that among the mollusc- 
oida (or mollusc-like creatures) geneagenesis seems to be the 
rule. 
We must refer the reader to the elegantly written work 
* Detailed iuformation concerning this subject will be found in Mr. 
Huxley’s paper on the “Organic lleproductions and Morphology of the 
Aphis,” tLinn. Trans.,’ 1858. 
I The Scolopendra, or “hundred legs,” common in gardens, is a 
myriopod ; the acorn barnacle so frequent on seaside rocks, is a cirrhi pede. 
