CRUSTACEANS 



421 



■SHELL GLANl} 



Order 4. — Leaf-footed Crustacea (Phyllopoda) 



These are Crustacea of varying size, though none are very 

 large, which are for the most part inhabitants of fresh water. 

 The segments and appendages differ greatly in number in different 

 cases, but those of the latter situated farther back than the head 

 are characterized by their flattened leaf-like form. There are two 

 sub-orders — i. Gill-footed Phyllopods (Branchiopoda), and 2. 

 Water- Fleas (Cladocera). 



I. Gill-footed Phyllopods possess a special interest, since they 

 not improbably present many of the characters distinguishing the 

 ancestral stock from which all the different groups of Crustacea 

 are descended. 



A typical genus is Apus (fig. 261), species of which are 

 sometimes found in great abundance at various localities on the 

 Continent, especially in flooded 

 meadows. For the sub-order 

 this form reaches a consider- 

 able size, being as much as 

 2 inches in length. It is of 

 a greenish -brown colour, and „!,„ 

 though segmentation is not 

 apparent in the head, the rest 

 of the body is composed of a 

 large number of obvious seg- 

 ments. The most striking fea- 

 ture is the presence of a broad 

 thin carapace, which, arising 

 from the head, extends back- 

 wards so as to cover the back 



and sides of the greater part of the body. The narrow posterior 

 end of the abdomen remains uncovered by the carapace, and 

 terminates in a pair of long jointed filaments. The head bears 

 a minute pair of antennules, vestiges of antennae, strong mandibles, 

 and two pairs of maxillae. A very large number of flattened 

 lobed swimming-feet are attached to the thorax and front part 

 of the abdomen. They are not limited in number to one pair 

 per body-segment, as is the case with the majority of Crustacea. 

 A typical swimming-foot consists of a number of small pointed 

 inner lobes, and two larger outer lobes, of which one is a soft 



Fig. 261. — Apus 



