72 
In certain Dragonflies (Calypteryx), and Hemiptera 
(Hydrometra), the legs, according to Brandt,* appear at a 
still earlier stage. 
According to the observations of Kollikert it would 
appear that in Donacia the segments and appendages 
appear simultaneously. Kolliker himself, however, admits 
that “ mez de hoc insecto observationes satis sunt manca,” 
and it is possible that he may never have met with an 
embryo in the state immediately preceding the appearance 
of the legs. 
On the whole, as far as we can judge from the observa- 
tions as yet recorded, it seems that in Homomorphous 
insects the ventral wall is developed and divided into 
segments before the appearance of the legs, but that the 
latter are formed simultaneously, or almost simultaneously, 
with the cephalic appendages, and before either the dorsal 
walls or the internal organs, 
As it may be interesting from this point of view to 
compare the development of other Articulata with that 
of insects, I give a figure (Fig. 32) representing one of 
the early stages in the development of a spider (Pholcus) 
after Claparede,t who says, “Cest 4 ce moment qu’ a 
lieu la formation des frofozorites ou segments primor- 
diaux du corps de lembryon. Le rudiment ventral 
s’épaissit suivant six zones disposées transversalement 
entre le capuchon anal et le capuchon céphalique. L’ceuf 
considéré par sa face ventrale offre alors un contour 4 peu 
prés circulaire et on peut le croire sphérique. Les zénes 
se montrent alors comme six circles d’un blanc plus 
éclatant, tracés sur la sphére.” 
Among Centipedes the development of Julus has been 
described by Newport.§ The first period, from the de- 
position of the egg to the gradual bursting of the shell, 
and exposure of the embryo within it, which, however, 
remains for some time longer in connection with the shelf 
by a distinct funis, lasts for twenty-five days. The seg- 
ments of the body, originally six in number, make their 
appearance on the twentieth day after the deposition of the 
egg, at which time there were no traces of legs. The 
larva when it leaves the egg is a soft, white, legless grub 
(Fig 33), consisting of a head and seven segments, the 
head being somewhat firmer in texture than the rest of 
the body. It exhibits rudimentary antennz, but the legs 
are still only represented by very slight papilliform pro- 
cesses on the undersides of the segments to which they 
belong. 
As already mentioned, I believe that at one time the 
vermiform state of the Homomorphous insects, which, as 
we have seen, is now so short, and passed through at so 
early a stage of development, was more important, more 
prolonged, and accompanied by a more complete condi- 
tion of the internal organs. The compression, and even 
disappearance, of embryonal stages which are no longer 
adapted to the mode of life, which do not benefit the 
animal, is a phenomenon not without a parallel in other 
parts of the animal and even of the vegetable kingdom. 
Just as in language long compound words have a tendency 
to concision, and single letters sometimes linger on, in- 
dicating the history of a word, like the “1” in “alms,” or 
the “b” in “debt,” long after they have ceased to in- 
fluence the sound; so in embryology useless stages, 
interesting as illustrations of past history, but without 
direct advantage under present conditions, are rapidly 
passed through, and even, as it would appear, in some 
cases altogether omitted. 
For instance, among the Hydroida, in the great 
majority of cases, the egg produces a body more or 
less resembling the common Hydra of our ponds, and 
known technically as the “trophosome,” which develops 
into the well-known Medusz or jelly-fishes. The group, 
however, for which Prof. Allman has proposed the term 
* Mem de !’Acad. Impé. des Sci. de St. Petersburg.” 1869. 
+ Observationes de Prima Insectorum Genesi, p. 14. 
t Recherches sur I’ Evolution des Araignées, 
§ Philosophical Transactions, 1847. Re 
NATURE 
aa ea a a a 
—LeeTn s i 
s 
q 
[May 22, 1873 
Monopsea,* and of which the genus AZgina may be taken 
as the type, is, as he says, “ distinguished by the absence 
of a hydriform trophosome, the ovum becoming developed 
through direct metamorphosis into a medusiform body, 
just as in the other orders it is developed into a hydriform 
body.” Figure 34 represents, after Allman, a colony of 
Bougainvillea fruticosa of the natural size. It is a British 
species, which is found growing om buoys, floating timber, 
&c., and, says Allman,t when in health and vigour, “ offers 
a spectacle unsurpassed in interest by any other species— 
every branchlet crowned by its graceful hydranth, and 
budding with Medusee in all stages of development (Fig. 
35), some still in the condition of minute buds, in which 
no trace of the definite Medusa-form can yet be detected ; 
others, in which the outlines of the Medusa can be dis- 
tinctly traced within the transparent ectotheque ; others, 
again, just casting off this thin outer pellicle, and others 
completely freed from it, strugzlinz with convulsive efforts 
to break loose from the colony, and finally launched 
forth in the full enjoyment of their freedom into the 
surrounding water. I know of no form in which so 
many of the characteristic features of a typical hydroid 
are more finely expressed than in this beautiful 
species.” 
Figure 36 represents the Medusa form of this species, 
and the development thus described may be regarded as 
typical of the Hydroida; yet, as already mentioned, the 
Eginidz do not present us with any stage corresponding 
to the fixed condition of Bougainvillea, but on the contrary 
are developed direct from the egg. 
But on the other hand there are groups in which the 
Medusiform stage becomes less and Jess important. 
Among the higher Crustacea again the great majority 
go through well-marked metamorphoses. Figs. 37 and 
38 represent two stages in the development of the prawn. 
In the first (Fig. 37), representing the young animal as it 
quits the egg, the body is more or less oval and unseg- 
mented, there is a median frontal eye, and three pairs of 
natatory feet, the first pair simple, the two posterior bira- 
mose. Very similar larvz occur in various other groups 
of Crustacea. 
They were at first regarded as mature forms, and O. F. 
Miiller gave them the name of Nauplius. So, also, the © 
second or Zoea form (Fig. 38) was at first regarded as a 
mature animal, until its true nature was discovered by 
Vaughan Thompson. 
The Zoea form of larva differs from the perfect prawn 
or crab in the absence of the middle portion of the body 
and its appendages, The mandibles have no palpi, the 
maxillipeds or foot-jaws are used as feet, whereas in the 
mature form they serve as jaws. Branchie are either 
wanting or rudimentary, respiration being principally 
effected through the walls of the carapace. The abdomen 
and tail are destitute of appendages.. The development 
of Zoea into the perfect animal has been well described 
by Mr. Spence Batef{ in the case of the common crab 
(Carcinus menas). 
All crabs, so far as we know, with the exception of a 
species of land crab (Gegarcinus), described by West- 
wood, pass through a stage more or less resembling that 
shown in Fig. 38. On the other hand the great group of 
Edriopthalma, comprising Amphipoda (shorehoppers, 
&c.) and Isopoda (woodlice, &c.), pass through no such 
metamorphoses ; the development is direct, as in the 
Orthoptera. It is true that one species, Tanazs Dulongii, 
though a typical Isopod in form and general character, is- 
said to retain in some points, and especially in the mode 
of respiration, some peculiarities of the Zoea type ; but 
this is quite an exceptional case. In Mysis, says F. 
Muller,§ “there is still a trace of the Nauplius-stage ; 
being transferred back to a period when it had not to 
* Monog. of the Gymuoblastic or Tubularian Hydroids. By_,G. J. 
Allman, F.R.S., &c., Roy. Society. tle, p. 315. - 
t Philosophical Transactions, 1859, p. 589. 
§ *‘ Facts for Darwin,” Eng. Trans., p, 127. 
