with Observations on the General Arrangement of the Artienlata. 



What takes place in regard to individual structural takes place alto in tin- 

 whole body, as is shown in the transformations of Inserts. While loou 

 ments of the body of an insect become more or less completely approximated 

 in sections, and divide the body into regions, the whole exhibit the same ten- 

 dency to approach each other, the head is applied more closely to the thorax, 

 and the thorax is approached by the abdomen. 



These, then, are the principles on which the body of an articulated animal 

 is developed, and acquires its proper form and dimensions, and which are 

 carried to their greatest extent in hexapod Insects. They seem to prepare 

 the way for a higher type of development at a much earlier period of the 

 ovum, of the Fertebrata, and to lead to the permanent division of the body, 

 in the more perfect animals, into important regions, — the head, thorax and 

 abdomen. 



This view of the principles of development of the body, and of the segment < 

 individually, will enable us better to understand the manner in which the most 

 important region of the whole animal, — the head, is constructed, to the ex- 

 amination of which I shall now proceed. 



Structure and Development of the Head. 



The Chilopoda, which have the head less completely formed than the Chi- 

 lognatha, apparently in consequence of their carnivorous habits, afford the 

 best means of tracing the construction of this part, and of observing the 

 changes and gradual approximation of the segments which enter into its com- 

 position. The number of segments that form the head of an articulated animal 

 has long been a matter of inquiry. Many attempts have been made to ascer- 

 tain the fact by examining the head in hexapod Insects, but in consequence 

 of its higher type of development and more compact form in that class, the 

 results arrived at by numerous inquirers are by no means uniform. The 

 different conclusions have, perhaps, arisen as much from the number of s. 

 ments that enter into its composition, as from the different species that have 

 been examined, in some of which every trace of some of the segments has 

 disappeared; while some portions are developed to excess in one genus, hut 

 are almost entirely atrophied in another. On this inequality depends the form 

 of the organ. Thus Burmeister recognises but two segments ; < aru.s and 



