The copulatory organs, the pet as ma, much be dealt with at length. In 191 3 

 K. Stephensen (The copulatory Organ (Petasma) of Sergestes vigilax (Stimps.) H. J. H., in: 

 Mindeskrift for Steenstrup, 191 3) shovved that the petasma begins to be visible in the Masti- 

 go/>7<s-stage and has a gradual development; he gives six figures to illustrate the development 

 and makes some remarks on them, but he did not undertake a more close comparative invest- 

 igation. For the rest, already in 1896 I had pronounced (op. cit.) that "the petasma itself does 

 not become completely developed at once to its final shape". But when Stephensen writes : 

 "As may be seen the petasma varies very much with the age, and thus may not be used as 

 a specific character", this conclusion is extremely wrong. In the order Euphausiacea the copul- 

 atory organs, to which also the not very necessary name "petasma" could be applied, show a 

 gradual development, but while it in the adult specimens is of extreme importance for the 

 separation of species — and Zimmer, Tattersall and others have foliowed me as to this nearly 

 fundamental question — I have as a rule not taken into account the lobes and processes in 

 not full-grown males, and at least in its earlier stages the petasma is too little developed. And 

 the state of things is the same in Sergestes. But in order to obtain good and practical results 

 it is necessary to adopt an appropriate plan as to the petasma in adult specimens and to carry 

 through a nomenclature for its essential elements. According to my experience from the Euphau- 

 siacea, I think that the left organ ought as far as possible to be chosen; when the animal is 

 seen from below consequently the petasma on the side turning to the right. This organ is cut 

 off, laid on a glass-plate in glycerine diluted with water, cautiously spread as far as possible 

 by the aid of two minute knifes, then a small glass-cover is applied in such a way that the 

 organ is not much pressed, and the organ is then drawn, consequently from behind (from below). 

 Furthermore the distal portion of its median and most important, ramified part ought to be 

 drawn with a higher degree of enlargement both from behind and, after the organ has been 

 turned, from in front. When each petasma of several species has been drawn with its lobes 

 turning upwards on the paper in the way described, the organs are easily compared and the 

 differences affording specific characters catch the eye. The right organ is completely symmetrical 

 with the left, but the latter ought to be preferred. It may be added that when one has made 

 himself acquainted with the petasma in a few species or when he has a number of satisfactory 

 figures at hand, he may as a rule find it unnecessary to remove the petasma from any male in 

 order to use it for determination, as the examination with a good pocket-lens will be sufficiënt. 

 The petasma consists of three main-parts, viz. pars media (PI. I, fig - . ia, m., fig. 2a,m., 

 fig. 6e, ;;/.), an inner plate, pars astringens (PI. I, a. in figs. ia, 2a, $è, ja; PI. II, a. in 

 fig. 2g), and an outer plate, pars externa (PI. I, fig. ia, e.). Pars astringens (a.) is always 

 a thin and generally rather large plate, which is fixed along the proximal portion of the inner 

 margin of pars media; the plate has generally a longitudinal folding, and its inner margin 

 which is long and straight, is equipped with a large number of extremely small hooks, by which 

 the plate of the left organ is coupled with the corresponding plate of the right one. Pars 

 externa (e.) projects from the proximal part of the outer margin of pars media ; it consists of 

 a rather large plate, lamina externa (PI. I, lam. in figs. ia, 2a, $6, 6e, ja, and PI. II, 

 lam. in fig. 2g) which anteriorly is produced into a generally long process, processus 



