COELOM. NEPHRIDIA. HEART. 317 



pair of nephridia, which thus furnish the opening to the exterior. 

 These ducts in the female retain a ciliated lining (Gaffron), 

 the only known instance of the occurrence of a ciliated tract 

 among the Arthropoda. 



The other groups of Arthropods present various stages of 

 modification of this primitive arrangement. In the Arachnida 

 segmented coelomic sacks (somites) are formed, and in their 

 walls the gonads are developed. In Scorpio five pairs of seg- 

 mental ducts appear in the embryo (in segments 3-6 and in the 

 8th *). Some of these disappear in the adult, but those of the 

 fifth persist, though with loss of the external aperture, as the 

 coxal glands, and those of the eighth as the ducts of the gonads 

 (Brauer). In other Arachnids a single pair of nephridia (cor- 

 responding to the fifth pair of appendages) also persists as the 

 coxal glands. 



In Myriapoda and Insects coelomic sacks are also developed 

 and from the walls of some of these the gonads are formed, but, 

 the excretory function having in these groups been in most 

 cases taken over by diverticula of the alimentary canal, no trace 

 of nephridia has been found. 



In Crustacea two pairs of nephridia persist, though they 

 are rarely found to coexist, as the antennal and shell glands, 

 but the mesoblast presents in this group the extreme of 

 differentiation from the annelidan arrangement, there being 

 scarcely any traces of its segmentation or of general coelomic 

 cavities. In all however the genital ducts are mesoblastic in 

 origin and it is thus open to us to regard them as derived from 

 a pair of nephridia, as analogy with Peripatus and Scorpio 

 would suggest. 



Connected with the presence of a haemocoelic body cavity in 

 place of the coelomic body-cavity of Annelids is another charac- 

 teristic feature of the Arthropoda, the relation of the heart to 

 the pericardial sinus. 



The heart in this group is a longitudinal dorsal vessel, per- 

 forated by one or more pairs of lateral ostia. These admit blood 

 from the pericardium, a special compartment of the system 

 of haemocoelic spaces which is separated from the rest by a 

 horizontal septum lying beneath it. 



* Counting, with Brauer, the cheliceral segment as the first. It is 

 however counted as the second (cf. p. 323) in the table on p. 525. 



