﻿NORTH AMERICAN NOTOSTRACA — LINDER 7 



BODY-RINGS AND POLYPODY 



Following the usual custom, I consider the body proper to begin with 

 the first postmaxillary ring, which is, however, strictly separated 

 from the head only on the ventral side. Packard (1883) counts the 

 first ten body-rings as the thorax, and the following ones as the ab- 

 domen. I include the eleventh body-ring in the thorax, as does Simon 

 (1886) in his important revision of the phyllopods and as Sars does in 

 his many descriptions of various notostracans. More than a hundred 

 years ago Zaddach (1841) showed, in an admirable work now almost 

 neglected, that the first 11 body-rings are essentially similar to each 

 other in the equipment of muscles, while the following ones differ 

 quite considerably from them. Further, the legs of this portion are 

 different, though the boundary in this respect is usually not distinct. 

 A boundary between different regions in this location is recognized 

 in crustaceans of many orders — so many, in fact, that it is not likely 

 to be a mere coincidence (Linder, 1941, p. 113). 



I have abandoned the use of the term "segment," replacing it with 

 the term "body-ring," when real rings or parts of rings are concerned. 

 I do this for the reason that in the abdomen there are no complete, 

 ordinary segments but two series of parts of segments, in some respects 

 independent of each other — the series of rings and the series of legs. 

 These are united to form a composite structure unique in appearance. 

 This is the true meaning of the much discussed phenomenon, poly- 

 pody. It is not an absolutely new view. Lankester (1904) expressed 

 a similar view, but did not give evidence for his opinion. However, 

 Zaddach (1841) had already supplied much evidence supporting that 

 opinion, and the following facts, most of which have not been noticed 

 by earlier authors, may give additional evidence : 



1. The series of legs covers a varying namber of body-rings in 

 specimens from one locality. Zaddach (1841) has given an example 

 of this. He also regards the number of pairs of legs as fixed; however, 

 I have found that this number is subject to a considerable variation. 

 The series of legs may stop at any place under a body-ring (fig. 20). 

 If we choose a certain pair of legs in the caudal part of the series 

 of legs, and determine its place under the series of body-rings, we shall 

 find that it has varying positions in various specimens from the same 

 locality (see table 2, p. 42). Thus, each body-ring in the abdomen 

 does not have a fixed number of pairs of legs, and this means that 

 there is no evident correlation, as is supposed by most authors, between 

 the series of legs and the series of abdominal body-rings. Evidence 

 for this fact from extra- American material is given in an earlier paper 

 (Linder, 1947). 



2. The boundaries between the body-rings in the leg-bearing part 

 of the abdomen do not continue ventrally. They stop at a longitudi- 



