NO. 14 



INSECT ABDOMEN— SNODGRASS 



39 



abdominal segement (somite XXVII), but provided with muscles 

 arising in both the second and the first segment. Each penis is armed 

 distally with a hook (B) mesad of which is the gonopore. In Poly- 

 artemia haseni the penes (C, Pen) are long tubular organs projecting 

 posteriorly from the under surface of the first and second abdominal 

 segments, which in this form are somites XXI and XXII. The 

 branchiopods, except in cases of parthenogenesis, propagate by sex 



Pen 



Fig. 14. — Crustacea-Branchiopoda-Anostraca: showing variable position of 

 penes. 



A, Branchipus scrratiis, with penes (Pen) arising _on somite XXVII. B, 

 same, end view of penes and gonopores (Gprs). 

 penes on somite XXII. 



C, Polyartemia haseni, with 



mating, and the spermatozoa are said to be discharged by the male 

 directly into the genital openings of the female ; few observations, 

 however, have been made on the exact manner of copulation and 

 intromission. 



The Malacostraca have a highly standardized body segmentation, 

 there being, with few exceptions, 18 somites between the mouth and 

 the telson. The trunk is consistently differentiated between seg- 

 ments XII and XIII into a cephalothoracic region and an abdomen ; 

 but the distinction between head and thorax is often somewhat con- 



