NO. 14 INSECT ABDOMEN SNODGRASS 13 



single organ at an early stage of its development. It is most unfortu- 

 nate that we know nothing of the genital openings of the trilobites. 



The great variation in the position of the genital outlets in different 

 groups of arthropods is only to be explained on the assumption that 

 quite different pairs of coelomic sacs have been utilized as genital 

 ducts. This assumption might seem to imply that the various arthro- 

 pod groups were differentiated from common ancestors that still 

 retained a full series of coelomic sacs with exit ducts ; but the idea 

 is entirely incompatible with the identity in structure of the gonads 

 in closely related groups that differ in the segmental position of the 

 gonoducts, as in Entomostraca, Chilopoda, and Hexapoda. More- 

 over, in the geophilomorphic chilopods the genital ducts may pertain 

 to different segments in different individuals of the same species, 

 according to variations in the total number" of somites formed, the 

 last somite being always the segment of the genital exit. The Chilop- 

 oda, the Protura, and the Collembola are alike in so far as the genital 

 opening in each sex occurs on the last somite, but there is a great dis- 

 crepancy in the number of somites between the genital segment and 

 the mouth, there being ii in Collembola, 17 in Protura, 21 to 30 in 

 most Chilopoda, and as many as 175 in the Geophilomorpha. In these 

 opisthogoneate groups new somites are not formed after the genital 

 segment is established ; in Hexapoda other than Protura and Col- 

 lembola one post-genital somite may be added (during embryonic 

 development) in the male, and four in the female. In the progoneate 

 myriapods the seventh postoral somite becomes the genital segment, 

 and an indefinite number of segments is generated in the postgenital 

 region. The genital segment of Chelicerata is always the eighth post- 

 oral somite ; in Malacostraca it is the tenth in the female and the 

 twelfth in the male, but in both these groups there is a fixed number 

 of postgenital segments. 



The recent proposal made by Tillyard (1935) that the genital seg- 

 ment is an identical primitive somite in all the Arthropoda, and that 

 its position in the definitive series of body segments is a matter of 

 whether other somites have been added before or behind it, entirely 

 disregards the evidence that somite formation in the arthropods pro- 

 ceeds always from a generative zone just before the telson. If telo- 

 blastic growth is a fundamental principle of development in the 

 Arthropoda, the segment of the genital ducts cannot be an identical 

 somite in all cases, and it is evident, therefore, that the genital outlet 

 segment is not necessarily predetermined as such by morphological 

 heredity. 



