NO. 14 INSECT ABDOMEN SNODGRASS 1 5 



utilized the primitive coelomic outlets, each should have a small ter- 

 minal part derived from the ectoderm. The primary gonoducts (fig. 

 4 A, Gd) are the vasa deferentia in the male, the ovidiictus laterales 

 in the female. Their external openings are the primary gonopores 

 {Gprs). The mesodermal ducts are usually provided with muscular 

 sheaths for the expulsion of the gametes, but with most of the arthro- 

 pods there has been added to the primary ducts, particularly in the 

 male, a more efifective exit apparatus formed of evaginations and in- 

 vaginations of the body wall at the mouths of the ducts, usually 

 accompanied by a special development of associated muscles. These 

 ectodermal parts of the exit system are highly variable, but, on the 

 other hand, they are often very similar in widely separated arthropod 

 groups, in which the gonopores are situated on quite different body 

 segments. Therefore, though it is convenient to employ a uniform 

 system of nomenclature for these parts so far as possible, the use of 

 the same terms in the description of the genital organs of different 

 groups should not be taken to imply anything more than a structural 

 or functional analogy in the parts named alike. 



One of the first steps in the development of ectodermal genital 

 accessories may consist merely of the formation of an external papilla 

 bearing the opening of each duct (fig. 4 B, Pen). Such structures are 

 usually termed penes in the male, whether they are functionally intro- 

 mittent organs or not, but they may be present also in the female, as 

 in Xiphosura. On the other hand, the mouth of each duct may be 

 carried inward at the end of a tubular ectodermal ingrowth (C, Dej), 

 which usually has a strong muscular sheath, and is termed a ductus 

 ejaculatorhis in the male, though the term would be quite as appro- 

 priate in the female. It is evident that the primary gonopores (A, 

 Gprs) are now at the inner ends of the ejaculatory ducts (C), but to 

 avoid a multiplicity of names the term gonopore is applied to the 

 opening of any genital duct, whether primary or secondary. A com- 

 bination of ejaculatory ducts with external penes is of common 

 occurrence (D). 



The primitive paired condition of the genital openings and associ- 

 ated structures is retained in some members of most of the arthropod 

 groups, and is characteristic of Xiphosurida. Crustacea, and Diplop- 

 oda. In other groups, and in some of the crustaceans and diplopods, 

 an unpaired condition of the terminal parts arises in various ways, 

 and is subject to great modifications, which reach an extreme degree 

 of development in the insects. In the first place, the penis may be a 

 single median organ (fig. 4 E, Pen) with two separate openings for 

 the ducts ; but in general there is associated with a median penis a 

 single ejaculatory duct (F, Dej), which receives the two vasa defer- 



