V CRUSTACEA— OUTER ORGANISATION 301 



carries 5 pairs of limbs, is known as the head, as apart from the trunk, 

 i.e. the whole of the remaining segmented body. 



The trunk of the JEntomostraca consists of a very varying' number 

 of segments, which in different regions may differ greatly in many 

 respects (heteronomous segmentation of the trunk). The trunk of the 

 Malacostraca always consists of a constant number of segments, viz. 

 fifteen. It always falls into two sharply distinguished regions, each 

 with a constant number of segments, an anterior thoracic region, 

 consisting of 8 segments, and a posterior abdominal region (pleon) 

 containing 7. 



The trunk of the Leptostraca {Nebalia), which in classification takes 

 a place half way between the conjectural racial forms of the 

 Entomostraca and those of the Malacostraca, though really more nearly 

 related to the latter, also consists of (1) a thorax of 8 segments 

 (which exactly answers to the thorax of the Malacostmca), and (2) an 

 abdomen of 8 segments. 



There is nothing to hinder us from assuming that in all Crustaceans 

 the segments which are numbered alike correspond, for instance the 

 2d, 6th, and 10th trunk segments of an Isopod with the 2d, 6th, and 

 10th trunk segments of a Phyllopod. 



As in the Annulata, so also in the Crustacea, the somites become 

 differentiated ontogenetically in regular order from before backward, 

 so that the youngest segment always appears posteriorly in front of 

 the anal segment, this latter containing the formative material for the 

 segments which are to appear in the course of development. There is 

 therefore also nothing to hinder us from assuming that the anal 

 segments of all Crustaceans correspond, of however many segments 

 the trunk may consist. 



Apart from the above-mentioned conjectural fusing of the 5 most 

 anterior primary segments to form the unsegmented head of the 

 Crustacean, the metamerism of the trunk, either of the whole trunk 

 or of single regions of it, may be obscured or even entirely obliterated. 

 Such obscuration or obliteration may in almost all cases be referred 

 to one or more of the following causes : — 



1. To the appearance of a shell or carapace as a fold arising from 

 the dorsal side of the posterior head region, which spreads in varying 

 form more or less far back over the body, covering or enveloping the 

 same. Such a carapace serves for the greater protection of the body, 

 and also is often closely connected with the respiratory functions. 

 Since a shield-like integumental fold is formed in an essentially 

 similar manner in the most various groups of Crustaceans and their 

 larval forms, we have reason for assuming that it represents a primi- 

 tive peculiarity of the Crustacean body. By the concrescence of the 

 shield or shell with the integument of all or a portion of the trunk 

 segments, the outer metamerism of the body is obliterated to a greater 

 or less extent. 



2. To the fusing of the anterior trunk segments with the head in 



