DIPLOPODA. 587 



which consists of a transparent projection of chitin covered 

 with fine sensory hairs and placed on the head near the base of 

 the antennae. The hypodermis cells below the projection 

 are specially modified and supplied by a special nerve from the 

 brain. 



The vascular system consists of a dorsal tubular heart con- 

 tained in a pericardium and branching into arteries in the 

 head. In each double segment there are two pairs of ostia, 

 one at each end of the segment, and it gives off from the middle 

 of each segment two (or one) pairs of arteries which pass 

 into the fat-body. 



The tracheae are unbranched in most Diplopods, but in the 

 Oniscomorpha they are branched. They arise in tufts from 

 tracheal pouches which open on the sternites immediately in 

 front of the articulation of the legs. There are therefore two 

 pairs of stigmata in each double segment and one pair in the 

 others. There are two kinds of tracheae : some larger and with 

 distinct spiral fibres, others smaller without the spiral fibre. In 

 .some Diplopods both kinds are present ; in others, one or the 

 other kind only. 



The excretory organs are represented by two or .four malpi- 

 ; ghian tubes which open into the gut at the junction of the 

 intestine and hind gut. It is possible that the cells of the fat-body 

 which contain urate of soda have also to do with excretion. It 

 has also been suggested * that the salivary glands, which as 

 Heathcote f has shown develop in Julus by the direct trans- 

 formation of a portion of the mesoblastic somite of the segment 

 of the gnathochilarium, may have the physiological as well as 

 ithe morphological characters of nephridia. 



The glandulae odori ferae are placed laterally, one pair in each 

 segment ; they open on the sides of the body by the foramina 

 repugnatoria which were formerly mistaken for tracheal stigmata. 

 In the Oniscomorpha the openings of the glands of the two sides 

 .are united, so that there is only one row of foramina repugnatoria 

 placed in the dorsal middle line. They begin usually in the 5th 

 or 6th segment and are not found in the apodal segments. 

 The secretion is often evil-smelling and contains prussic acid. 



* Bruntz L., Contribution a 1'etude de 1'excretion chez les Arthropodes, 

 Arch. Biol, 20, 1903, p. 217. 

 t Phil. Trans., 1888, p. 160. 



