232 CRUSTACEA. 



of the hypothetical ancestor of the Entomostraca, must be regarded 

 as decidedly degenerate. Among these we must reckon the small 

 size of the body and the comparatively small number of its 

 segments, the reduced form or entire absence of the heart, the 

 want of separate respiratory organs (branchiae), the loss of the 

 paired lateral eyes, which are retained only in the families Cory- 

 caeidae and Po?itellidae, and perhaps also the slight development of 

 the dorsal shield. On the other hand, there are certain indications 

 that the Copepoda ought to be counted among the most primitive 

 of existing Crustacea. Among these characters we should specially 

 mention the use of the two pairs of antennae as locomotory and 

 clasping organs, the very primitive structure of the mouth-parts in 

 the free-living forms (occurrence of a biramose mandibular palp, the 

 segmentation of the first maxilla, Fig. 91 A, p. 194), and the meta- 

 morphosis which, in the free-living forms, shows very primitive 

 features. 



With regard to the segmentation of the body, we must distinguish 

 as the most anterior region of the body a simple cephalic region 

 carrying the antennae and the mouth-parts. The latter consist of 

 three pairs of appendages (mandibles, first and second maxillae), the 

 last pair separating into two appendages (the exopodite which shifts 

 forward is called the first maxillipede, the endopodite which shifts 

 backwards yields the second maxillipede). The thoracic region con- 

 sists of five segments provided with biramose swimming limbs (Fig. 

 90 A, p. 193); the last of these segments may be vestigial, -whilst 

 the anterior segment often fuses with the cephalic segments, this 

 union giving rise to an anterior region known as the cephalo-thorax. 

 The abdomen consists of five segments, the most anterior of which 

 alone carries the rudiment of a limb (genital prominence). A fusion 

 of the two anterior abdominal segments usually gives rise, in the 

 female, to a double genital segment which bears the genital aperture. 



In a few Pontellidae, the cephalo-thoracic region, by the appearance of 

 demarcations between the segments, becomes subdivided into regions (each 

 consisting of two segments). This peculiarity stands almost alone in the whole 

 series of Crustacea. Such a segmentation can only be regarded as a secondary 

 re-appearance of the long-lost independence of the cephalic segment. It has, 

 nevertheless, a certain interest. 



A. Gnathostomata. 

 The metamorphosis of the free-living Copepoda is accomplished as 

 a very gradual transition from the Nauplius to the i adult form through 

 many moults. There is, however, at a certain period, a more sudden 





