336 INSECT A. 



invagination as a structure mi generis. We may here point to the condition 

 of the terrestrial Isopoda, in the branchial lamellae of which air-containing 

 spaces altogether analogous to tracheae develop {Tylus). 



F. The Alimentary Canal and Intestinal Glands. 



Of the three sections of the alimentary canal, the stomodaeum, 

 enteron, and proctodaeum, the first and third arise as ectodermal 

 invaginations. In most cases the rudiment of the stomodaeum 

 appears in the germ-band somewhat earlier than that of the procto- 

 daeum (p. 294 and Fig. 145 C, m, p. 291). The musculature of 

 these sections is yielded by the surrounding mesoderm. In the 

 stoinodaeal invagination an unpaired dorsal depression soon appears, 

 from which are derived the frontal ganglion and the oesophageal 

 nerve (p. 328). 



The actual ectodermal character of the fore- and hind-guts has been established 

 with considerable certainty by the unanimous testimony of observers, and by 

 comparison with the conditions in other groups of Arthropoda. Voeltzkow 

 (No. 85), indeed, has recently derived both structures from the lower layers, 

 and Graber has adopted this view for the proctodaeum of Musca. With 

 regard to this, we must refer to what has already been said (p. 315) as to the 

 condition of Musca. 



The connection between the cavities of the stomodaeum and proctodaeum 

 with that of the enteron is usually established at a somewhat early embryonic 

 stage. In certain larval forms, however (many Hymenoptera, e.g., Apis and 

 Myrmeleon), no communication is established between the enteron and the 

 proctodaeum, the latter then having an exclusively excretory function. 



The enteron develops from two originally distinct rudiments, the 

 anterior and posterior entoderm-rudiments (p. 313), which from the 

 very first bear a close relation to the invaginations of the stomodaeum 

 and proctodaeum. Although originally applied as simple cell-accumu- 

 lations to the inner ends of these invaginations, so closely, indeed, 

 that Voeltzkow (No. 85, 86), Patten (No. 68), and Graber 

 (Nos. 28 and 30) derived them directly from the epithelium of 

 the latter, they soon extend, through continuous cell-proliferation, 

 and assume the U-shape (Fig. 153 A, en' and ere").* The ends of the 

 U-shaped rudiment in the anterior entoderm-mass are directed 

 posteriorly, but in the posterior mass anteriorly. These ends grow 

 out towards each other, meet and fuse, and thus form two paired 

 entoderm-bands running dorsal to the germ-band along its whole 

 length. 



* [As already stated (footnote, p. 317), Heymons (No. XX.) and Lecaillon 

 (No. XXIX.) consider that the entoderm is quite wanting in the adults of the 

 higher Insects, the mid-gut originating, according to these observers, as ecto- 

 dermal ingrowths from the stomodaeum and proctodaeum. — Ed.] 



