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 Podophthaknous Crustacea it is clearly seen to be composed of six 

 somites, each possessing a pair of appendages; of these, the fii'st are the eyes ; the second, 

 the antennules ; the tliird, the antennae ; the foiu-th, the mandibles ; the fifth, the first 

 maxillae ; and the sixth, the second maxillae. 



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 pan* of appendages are antennae ; the second pair, mandibles, with a hugely deve- 

 loped palpvis ; the third paii', first maxillae ; and the foiu'th paii", second maxillae, converted, 

 like the next two pairs of appendages, into ambulatory legs. 



It follows, therefore, if we take the number of moveable ajipendages 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 Edrioj)hthalmia. But by this 

 assumption we arrive at a stiU closer approximation of the different classes in regard to 

 their cephalic structm*e ; 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 fm-ther, and add to this the hypothesis, that in the Articulata 

 the head is normally composed of six somites, which are aU fully developed only in 

 Fodophthalmia, 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 thii-d unde- 

 veloped. Admitting this hypothesis, I find further, that of the six cephalic somites, the 

 sterna of three (the mandibular and two maxillary) are alwaj'S situated behind the mouth 

 and on the ventral surface of the body. The position of the other three varies ; but the 

 most anterior or ophthalmic is always bent vipwards 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 every 

 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 Myri<ipoda 

 the total number of somites even is suscejjtible of an exti'cme amount of variation. But 

 in the other classes it appears to me that there is a tj^ical number of somites, from whence 



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