ARTHROPODA 265 



(ii.) The Eucopepoda are further subdivided into : (a) the, 

 Gnathostomata or free-swimming forms with masticating mouth 

 appendages ; and (b) the Parasitica, in which the mouth parts 

 are modified for sucMng or piercing, and whose body is incom- 

 pletely segmented. 



(a) Gnathostomata. — The genus Cyclops, which includes 

 many species living in fresh or brackish water in Great Britain, 

 is fairly typical of this subdivision. In shape its body has been 

 compared to that of a split pear with its flat side ventral, 

 tapering posteriorly. The head bears five pairs of appendages, 

 viz. (i.) antennules, (ii.) antennae, (iii.) mandibles, (iv.) 1st 

 maxillae, (v.) 2nd maxillae. Dorsally its terga form a con- 

 tinuous carapace, and are fused with the first thoracic tergum. 

 The thorax consists of six segments, the five posterior being 

 free dorsally as well as ventrally, except in the female, where 

 the last thoracic segment is fused with the first abdominal. 

 The abdomen consists of four cylindrical segments devoid of 

 appendages, the last bears the furcal caudal processes so 

 characteristic of the Entomostraca. 



Appendages of Cyclops. 



1. Antennules. 5. 2nd maxillae (maxlllipedes). 



2. Antennae. 6-9. Four pairs of thoracic limbs. 

 3! Mandibles. 10. 5tli pair „ ,, 



4. 1st maxillae. (rudimentary). 



The antennule is a well- developed appendage, which acts 

 as a strong oar, in the male it also acts as a clasper, and 

 is correspondingly modified; the antennae are short and 

 four-jointed; the mandibles guard the mouth, one on each 

 side; they and the 1st maxillae have rudimentary palps. 

 The 2nd maxillae are biramous, the split extending to their 

 base; the shell gland or excretory organ opens upon the 

 first joint of the outer ramus. The first four thoracic limbs 

 are flattened from before backward; they are biramous, and 

 their bases are united by a median plate, which extends across 

 the middle ventral line. The fifth thoracic segment bears a 

 pair of rudimentary feet at the sides of the ventral surface. 

 The next segment has a rounded cross section Hke those of the 

 abdomen, with which it is generally grouped; but as it bears 

 the openings of the genital ducts, it is better to regard it as 



