THE TRACHEAE — THE PROTECTIVE GLANDS. 243 



The Tracheae. 



The tracheae develop late. In Geophilus, the embryo has no 

 tracheae when it leaves the egg, and respiration at first takes place 

 through the covering of the body, which is still very thin (Sograff). 

 In the Diplopoda also, the tracheae appear late, apparently only after 

 the embryo has split the chorion, and is lying in its pupal integument. 

 In Julus, there is at this stage, behind the base of each leg, a pit-like 

 depression which deepens and forms two diverticula, one of which 

 extends into the space beneath the ventral chain of ganglia, while 

 the other grows out dorsally. These are the two principal trunks 

 proceeding from the tracheal sac, and give rise to the tracheal tubes 

 Avhich are lined with a chitinous intima. 



In the Diplopoda, each trunk-segment has two pairs of stigmata, 

 •each leading into a tracheal sac, from which originate bundles of 

 simple tracheal tubes. The tracheal tubes do not branch, nor do 

 they anastomose ; they thus exhibit a very primitive form. The 

 presence of two pairs of stigmata on each segment in the Diplopoda 

 is an additional reason for regarding the segments as arising by the 

 fusion of two originally distinct somites. 



The tracheae of the Chilopoda are more complicated, as they form branches and 

 anastomose. The distribution of the stigmata over the segments is no longer so 

 regular, though, as a rule, a single pair open on the pleural membrane of most 

 segments, but in some segments they are wanting. Scutigera has only a single 

 median dorsal stigma on each segment ; in the Symphyla there are only two 

 stigmata, which are situated on the head. In this respect the Chilopoda appeal' 

 as the less primitive forms. 



The Protective Glands. 



In the Diplopod larva with three pairs of legs, a pair of depressions 

 appear laterally on the fifth trunk-segment ; these are the foramina 

 repugnatoria which lead into flask-shaped ectodermal invaginations 

 representing the first pair of stink- or protective glands (Fig. 122, id). 



As the segments increase in number, more of these glands develop ; 



each of the double segments contains one pair only, they are never 



found in the anterior single segments (Heathcote, Metschnikoff).* 



compound eye. The retinulae have been greatly reduced in number, and each 

 has become elongated in the plane of the depression of the original ocellns. 

 Two sets of these retinnlae can be recognised. A lower set, consisting of three 

 cells only, forms an elongate trifoliate stalk to the ocellus, the rhabdoms of 

 these three cells being correspondingly elongated and in contact with one 

 another. The outer set are more numerous, consisting of twelve cells whose 

 rhabdoms are widely separated by the development of a crystalline cone formed 

 by the fusion of five to seven cells, which are perhaps the representatives of 

 the vitreous body. The whole is covered by a cuticular lens. The eye of 

 Scutigera is regarded as intermediate in character between the simple and the 

 facet-eye, and has been termed a pseudo-facetted eye. — Ed.] 



* For the interpretation of these glands we refer the reader to p. 252. 



