Smithsonian miscellaneous collections . body, but more generally they occur on the organs are present in some groups, as in Branchiopodaand Ostracoda, the males of which may have a pair of penes, or asingle penis through which both vasa deferentia discharge. Examplesof the paired type of organ are shown here in two branchiopodspecies, Branchipus serratus (fig. 14 A, B) and Polyartcmia haseni(C). The two penes of the first species are short, thick papillae(A, Pen) projecting ventrally from the under surface of the second NO. 14 INSECT ABDOMEN—SNODGRASS 39 abdominal segement
Smithsonian miscellaneous collections . body, but more generally they occur on the organs are present in some groups, as in Branchiopodaand Ostracoda, the males of which may have a pair of penes, or asingle penis through which both vasa deferentia discharge. Examplesof the paired type of organ are shown here in two branchiopodspecies, Branchipus serratus (fig. 14 A, B) and Polyartcmia haseni(C). The two penes of the first species are short, thick papillae(A, Pen) projecting ventrally from the under surface of the second NO. 14 INSECT ABDOMEN—SNODGRASS 39 abdominal segement (somite XXVII), but provided with musclesarising in both the second and the first segment. Each penis is armeddistally with a hook (B) mesad of which is the gonopore. In Poly-artemia haseni the penes (C, Pen) are long tubular organs projectingposteriorly from the under surface of the first and second abdominalsegments, which in this form are somites XXI and XXII. Thebranchiopods, except in cases of parthenogenesis, propagate by sex. Pen Fig. 14.—Crustacea-Branchiopoda-Anostraca: showing variable position ofpenes. 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 maledirectly into the genital openings of the female; few observations,however, have been made on the exact manner of copulation andintromission. The Malacostraca have a highly standardized body segmentation,there being, with few exceptions, 18 somites between the mouth andthe 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- 40 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 ventional, and when there is a well-marked anatomical separation theboundary is not the same in all cases. Carcinologists usually de
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