520 THE INVERTEBRATA 



in Other respects. It may be pointed out, however, that there is also 

 a connection between the degree of development of tracheae in a 

 family and the activity of its members. In inert forms, there may be 

 reduction or even total loss of the tracheal system. 



In all the forms in which lung books or gill books are present, there 

 are processes in the embryo which can be identified as rudiments of 

 appendages, on the anterior abdominal segments (Fig. 352 D). On 

 the posterior border of these processes, leaflets develop at the same 

 time as an invagination forms the lung cavity above them, so that the 

 limb itself forms part of the floor of the cavity. On the whole then, 

 embryology may be said to show the origin of lung books from gill 

 books, and the comparative anatomy of spiders indicates that lung 

 books have been replaced by tracheal systems. But there lie outside 

 this series arachnid groups, like the Acarina, with tracheal systems 

 of a different kind, which can only be derived with difficulty from the 

 respiratory system of the other forms and may have had a separate 

 origin. 



In the arachnids, the mesoblast is formed as two lateral bands 

 which segment into somites, just as does the same tissue in the anne- 

 lids. The somites correspond with the external segmentation and in 

 each one of them appears a coelomic cavity. This is best seen in the 

 scorpions (Fig. 352 C) and the spiders (Fig. 352 B). They are formed 

 near the ventral surface and extend on the one hand into the appendage 

 and on the other towards the dorsal middle line, where the extensions 

 from the two sides meet and form the heart between them. They also 

 form diverticula varying in the different groups, which are the remains 

 of a complete series of metamerically segmented coelomoducts. In the 

 scorpions, the embryo (Fig. 352 C) shows five pairs of these, in seg- 

 ments 3, 4, 5, 6 and 8. In only one case, that of segment 5, do the 

 coelomoducts reach the external surface, and persist in the adult as a 

 pair of excretory organs, the coxal glands. In segment 8 they grow 

 towards the middle line and form the mesodermal part of the gono- 

 ducts. The other coelomoducts disappear and the coelomic sacs are 

 resolved into mesenchyme which fills up the spaces of the body and 

 forms the muscles, the blood and the fat body. In Limulus there are 

 also a pair of coxal glands, which in development arise from the 

 coelomic somites of no less than six segments, of which only segment 

 5 sends out a duct opening to the exterior. 



Class SCORPIONIDEA 



Arachnids with the prosoma covered by a dorsal carapace; the 

 opisthosoma divided into a mesosoma and metasoma distinct from 

 one another, containing twelve segments and a telson; chelicerae 



