with Observations on the General Arrangement of the Articulata. 279 



External Anatomy. 



Before I proceed to describe the species of Myrlapoda, it is necessary that 

 some notice should be taken of the general external anatomy of the class, in 

 order that we may be enabled better to understand the analogies and to trace 

 the origin of the structures which enter into the composition of the head, that 

 portion of the animal from which most of the characters of the genera are 

 derived. The laws which govern the development of this region of the body 

 in the J^r/a/>oc?a regulate not only the whole form of body in all the Articu- 

 lata, and the changes from that of the elongated, cylindrical, segmented larva, 

 to the compact, highly organized and complicated body of the perfect insect, 

 but they regulate also the form of body and development of every structure in 

 the f^ertebrata, even including those of oiu' own system. 



It is on the varied extent to which some entire segments, or certain portions 

 of some of the segments, are developed in the embryo or in the larva, — more 

 especially those in the anterior part of the body, — that the peculiar form of 

 every articulated animal depends. 



Every segment of an articulated animal is a compound structure. The 

 identity of those parts which enter into the composition of the segments of the 

 thorax of hexapod Insects has already been admirably pointed out by Professor 

 Audouin. The principles on which the changes take place, and the analogies 

 which the appendages of the head bear to those of the other regions of the 

 body, had previously been shown in the excellent researches of Savigny. My 

 object, therefore, is now to develope still further the principles already laid 

 down by these authorities, and to point out the existence of parts in the ske- 

 leton of the Myriapoda analogous to those which have been shown in Insects ; 

 and also to offer some additional proofs of the universality of the great laws 

 of development by the aggregation and coalescence of contiguous structures, 

 as shown most distinctly in the aberrant class, Myriapoda. 



Naturalists are aware that in all the Articulata the organs of support are 

 on the exterior surface of the body, and consist entirely of certain portions of 

 the tegument in which earthy matter is deposited and consolidated. This 

 dermo-skeleton affords an unyielding surface of attachment for the muscles, 

 and more or less completely encases the whole body. In the higher Articu- 



