320 . INSECTA. 



that the boundaries of the consecutive primitive segments which are marked 

 by dissepiments do not always exactly coincide with those of the segments 

 of the germ-band. This is especially noticeable in later stages, and is caused 

 by the former shifting a little in position. 



As a rule, each true segment of the primary trunk has a pair of primitive 

 segments. Besides these, a pair of coelomic sacs develops in the primary 

 cephalic region in Blatta (Cholodkowsky, No. 19), in Stcnobothrus and Mantis 

 (Graber, No. 30). These sacs would correspond to the cephalic cavities in 

 Peripatus (p. 199). The Orthoptera also appear to have a pair of coelomic sacs 

 in the terminal segment (Cholodkowsky). In Hydrophilus, on the contrary, 

 the coelomic sacs are not only wanting in the cephalic and anal regions, but 

 appear suppressed in the mandibular segment, and their development is delayed 

 in the first maxillary segment (Heider). 



The coelomic sacs vary greatly in size in the different groups 

 of Insects. They develop to the greatest extent in the Orthoptera 

 (Fig. 157), in which almost the whole cell-material of the meso- 

 derm is used up in their formation, and in which, according to 

 Cholodkowsky (No. 19), Graber (Xo. 30), and Heymoxs (No. 43), 

 the conditions under which the coelom develops bear considerable 

 resemblance to those described in connection with Peripatus (p. 199 

 et seq.). The very extensive cavities of the primitive segments which, 

 in the Orthoptera, reach into the limb-rudiments also (Fig. 157 B, ex), 

 at a later stage are broken up by constrictions into dorsal and ventral 

 halves (Fig. 157 B, c, c"). The ventral halves (<•'), which extend 

 into the limb-rudiments, soon degenerate (Fig. 157 C), the cells 

 of their walls losing their epithelial continuity and becoming 

 irregularly grouped like a mesenchyme. The shifting apart of these 

 cells and their separation from the surface of the food-yolk then 

 gives rise to the permanent body-cavity. The dorsal halves of the 

 cavities of the primitive segments, on the contrary, are long retained 

 (as will be seen below, p. 338) and play an important part in the 

 development of the layer of intestinal fibres, the heart, the peri- 

 cardial septum, and the genital organs. 



In the higher groups of Insecta (the Coleoptera, Lepidoptera, and 

 Hymenoptera), the primitive segments no longer form on such an 

 extensive scale (Fig. 158 D-F, us). They are here comparatively 

 small sacs lying in the lateral parts of the germ-band, and correspond 

 to the dorsal parts only of the coelomic sacs of the Orthoptera. The 

 ventral parts are here from the very first replaced by mesenchyme. 

 There are consequently, in these forms, no coelomic diverticula in 

 the limb-rudiments. 



In the Muscidae, the development of coelomic sacs is apparently completely 

 suppressed (Graber, No. 28). We find their equivalent here in diverticula, 



