422 CHARACTERS OF INVERTEBRATE ANIMALS 



pear-shaped gill. Such a limb is regarded by many as the 

 generalized type from which the varied kinds of appendages 

 found within the class have been derived by modifications of 

 different kinds (see p. 403). This may perhaps be so, but it is 

 not always easy to recognize with certainty the equivalent parts 

 in different Crustacean groups. All these swimming-feet in Apus 

 are by no means exactly alike; e.g. some of the inner lobes of the 

 first thoracic pair are drawn out into long filaments, liable at first 

 glance to be mistaken for antennules and antennae, while in the 

 female the eleventh thoracic pair are partly modified into brood- 

 capsules within which the eggs develop. 



The excretory organs of Apus are not, as in the higher 

 Crustacea, antennary glands, but coiled tubular shell-glands which 

 open to the exterior upon the second maxillae. Three eyes are 

 placed close together on the top of the head — a small unpaired 

 " nauplius eye " in the middle line, and a pair of compound eyes. 



2. The Water-Fleas (Cladocera) are small active Crustacea, 

 mostly inhabiting fresh water. The characters of the sub-order 

 are well seen in the very common form Daphnia (fig. 260). The 

 thorax and shortened abdomen are enclosed in a bivalve carapace 

 which grows back from the head. The usual five pairs of 

 appendages are borne by the head (except second maxillae, which 

 are absent), and of these the most remarkable are the large forked 

 plume-like antennce which are used as swimming organs. There 

 are five pairs of flattened thoracic swimming-feet, something like 

 those of Apus, but the short abdomen is limbless. It bends 

 sharply round to the under surface and ends in a pair of curved 

 bristles. When Daphnia is examined alive under the microscope, 

 an oval pulsating sac will be noticed near the dorsal surface. 

 This is the heart. The coiled shell-gland is also easily seen, and 

 the large compound eye is a conspicuous object. It lies in the 

 front of the head, and has been formed by the fusion of two lateral 

 eyes. It is constantly in a state of trembling movement. A small 

 nauplius eye is present a little farther back. 



Class 6.— KING-CRABS (Xiphosura) 



This small but interesting class includes only the King-Crab 

 {Limulus), a large marine Arthropod living in shallow water in 

 the East Indies and the warmer parts of the West Atlantic and 



