THE ARTHROPODA. 423 



of the two sides becoming detached and uniting to form the dorsal 

 tube. The primitive segments then break up to some extent, single 

 cells from various parts wandering into the primary body-cavity and 

 there forming a kind of mesenchyme. The permanent body-cavity 

 arises through the appearance of lacunae in this mesenchyme and 

 the flowing together of these lacunae to form large spaces. As a 

 last remains of the primitive segments we have the pericardial 

 septum so characteristic of the Arthropoda which cuts off a dorsal 

 part of the body-cavity surrounding the heart (the pericardial space) 

 from the larger ventral portion. 



Besides the parts just mentioned, the primitive segments yield 

 the formative material for the nephridia. In Peripatus, where the 

 nephridia appear, as in the Annelida, in all the trunk-segments, 

 a considerable portion of the primitive segments is directly utilised 

 for the formation of the nephridia. In the other groups, the whole 

 question of the rise of the organs known as nephridia is still 

 undecided, but it may be mentioned as very probable that the 

 salivary glands and anal glands of Peripatus, the antennal and shell- 

 glands of the Crustacea, the coxal glands of Limulus and the 

 Arachnida, as well as the efferent genital ducts, are derived from 

 nephridia, and in any case are mesodermal in origin. The nephridial 

 nature of the organs of Peripatus just mentioned, and of the antennal 

 and shell-glands of the Crustacea, may be regarded with some 

 certainty as definitely established. The aperture of the efferent 

 genital ducts varies greatly in position in the different divisions of 

 the Arthropoda. From this we may conclude that, in the different 

 cases, the nephridia of different segments have been drawn into the 

 formation of the genital ducts (Chilopoda and Diplopoda), although, 

 in some cases, the idea of a secondary shifting of the opening through 

 several segments is suggested. (In the Insecta, the apertures of the 

 genital organs vary from the seventh to the ninth segment. In 

 the Ephemeridae, the female genital aperture is found on the seventh 

 segment, while, in other Insects, it lies behind the eighth segment.) 



The genital glands also are derived from the primitive segment.-, 

 these being found as growths of the epithelium of the coelomic sacs, 

 and thus having an origin exactly similar to that of the genital glap^s 

 of the Annelida (Vol. i., p. 297). A further agreement with the 

 Annelida is found in the fact that the remains of the coelomic 

 sacs may pass direct into the cavity of the genital gland, so that 

 the genital products budding off from the periton<\il epithelium can 

 still fall into the secondary body-cavity (coelomic or genital cavity), 



