470 M. straus-durckheim's 



besides the head. In the larvce they are, as in the Myria- 

 poda, nearly similar ; but they undergo great changes in the 

 metcmiorphosis, becoming divided into four principal parts, 

 namely, the head, the corselet {prothorax), the thorax, and 

 the abdomen; the first and two last are also composed of more 

 segments than one, united. It appears, from a comparison of 

 the head of Coleoptera with that of the Scolopendra inorsi- 

 tans, that the head of insects is formed by a union of seven 

 segments, represented by the lahrum, clypeus, epicranium, 

 and inandibulce, labium, prebasllar, and the two last by the 

 basilar, of which the appendages form the maxillce. Perhaps 

 the labrum or clypeus forms part of the segment to which the 

 lahiiwi belongs, and the epicranium may also be part of the 

 segment of which the prebasilar forms part ; in this case the 

 head consists but of five segments.*^ 



The Crustacea present more numerous modifications in the 

 form of the body, and less sudden transitions from one genus 

 to another, than the Myriaptoda and insects ; and here, as well 

 as in the Arachnida, the change of the feet to organs of man- 

 ducation is so clear as to leave no room for doubt. Many genera 

 of the Jsopoda have the body similarly formed to the Myria- 

 poda, except that the last segments undergo some modification 

 on account of the change of the feet into respiratory organs. 

 The head is distinct, and probably composed of several 

 segments. In the Amjyhipoda the posterior segments dimi- 

 nish in bulk, marking out the distinction between the trunk 

 and abdomen, which becomes complete in the Decapoda, 

 where the segments of the trunk are intimately united, and 

 where the anterior segments constitute a head confounded 

 with the trunk, whilst the posterior remain moveable, dimi- 

 nishing gradually from genus to genus until they are reduced 

 to mere rudiments in the Brachyura. Lastly, in Limulus, 

 the head has disappeared, and the food is masticated solely by 

 the feet. The body is divided into two segments, the trunk 

 and abdomen ; but the distinction of their respective segments 

 is scarcely visible, except the last, which is styliferous. We 

 now arrive at the Arachnida, the organization of which in 

 general nearly resembles that of Limulus. The body is com- 

 posed, according to the families, of two or three parts. In 

 Thelyphonus the trunk bears the members and the mouth ; 



■^ Ctrtaiiily not a correct view of the structure of the head. — D. 



