AND MORPHOLOGY OF APHIS. 229 
6. In Insecta and Crustacea the head, in the embryo, is easily distinguishable from the 
rest of the body. In Podophthalmous Crustacea it is clearly seen to be composed of six 
somites, each possessing à pair of appendages; of these, the first are the eyes; the second, 
the antennules; the third, the antennæ; the fourth, the mandibles; the fifth, the first 
maxillæ ; and the sixth, the second maxillæ. 
In Insecta, on the other hand, only four pairs of appendages appear in the head, the 
eyes being sessile, and one pair of antennary organs remaining undeveloped. 
In the Arachnida it appears to me to be quite clearly shown by development that the 
anterior pair of appendages are antennæ ; the second pair, mandibles, with a hugely deve- 
loped palpus; the third pair, first maxillæ ; and the fourth pair, second maxillæ, converted, 
like the next two pairs of appendages, into ambulatory legs. 
. It follows, therefore, if we take the number of moveable appendages as the test, that in 
the Articulata never more than six, and never fewer than four somites enter into the com- 
position of the head. But is the number of moveable appendages a just test of the num- 
ber of somites entering into a part? No one will pretend that it is so in the abdominal 
and thoracic regions; and if we consider the head of Crustacea alone, we find the eyes 
becoming sessile, and one pair of antennary organs aborting, without the least reason for 
concluding that the typical structure of the head is altered. It seems to me, then, hardly 
a hypothesis to assume that the sessile eyes of Insects represent the appendages of a 
somite, since it is universally admitted that they do so in Edriophthalmia. But by this 
assumption we arrive at a still closer approximation of the different classes in regard to 
their cephalic structure; for all would, on this supposition, have either five or six cephalic 
somites,—the former number being invariably met with in the true air-breathers (though 
in many purely aquatic forms also), while the latter is found only in those which respire 
by means of gills. 
I repeat, I can see nothing in this generalization but a simple expression of the facts. 
But I would go a step further, and add to this the hypothesis, that in the Artieulata 
the head is normally composed of six somites, which are all fully developed only in 
Podophthalmia, Stomapoda, and some Branchiopoda; while in other Crustacea, some one 
or more of the pre-oral somites is more or less abortive, and in Arachnida and Insecta, 
the appendages of the first somite are sessile, and those of the second or third unde- 
veloped. Admitting this hypothesis, I find further, that of the six cephalic somites, the 
sterna of three (the mandibular and two maxillary) are always situated behind the mouth 
and on the ventral surface of the body. The position of the other three varies 3 but the 
most anterior or ophthalmic is always bent upwards in consequence of the cephalic flexure, 
and not unfrequently, as in Insects, constitutes the greater part, or the whole, of the dorsal 
region of the head. The next two, or antennulary and antennary sterna, may present cray 
variation from approximative parallelism with the axis, in Squilla, to extreme reflexion, as 
in Insecta and many Crustacea. : 
7. Nothing can be more variable than the number of the somites whence appendages 
are developed in the various classes and orders of the Articulata; and in the Myriapoda 
the total number of somites even is susceptible of an extreme amount of variation. But 
in the other classes it appears to me that there is a typical number of se 43 whence 
H 
