COPEPODA 743 



All of these appendages are built on the same plan, which is 

 typically represented in the swimming feet of Cyclops (Fig. 1221). 

 Each foot consists of two basal segments, and attached to the outer 

 or distal of these are two branches or rami, each of three segments.. 

 The outer ramus is known as the exopodite and the inner as the 

 endopodite. This typical plan may be very much modified but, 

 in most cases the fundamental structure can be recognized. 



Of these appendages, the first antennae are very characteristic. 

 They are so modified that one of the rami has entirely disappeared, 

 and the one remaining is made up of a considerable number of 

 segments, varying from six to twenty-five. In the same species 

 the number of segments in the antennae is ordinarily invariable. 

 In some of the species of Cyclops the antennae are very short, in 

 others they may exceed the length of the cephalo thorax, while in 

 the other genera they may equal or exceed the length of the whole 

 body. The segments of the antennae are armed with hairs which 

 are definite in number and location. They have also sensory 

 structures arranged in definite places on the segments. The club- 

 shaped sensory appendage of the twelfth antennal segment of 

 some of the species of Cyclops is one of the important means of 

 identification. Some of the species of Cyclops have circlets or 

 crowns of spines on certain antennal segments which give them a 

 peculiarly ornate appearance. In some of the species of Cyclops 

 there is a thin hyaline lamella extending longitudinally along cer- 

 tain of the segments, being especially marked on the last two. This 

 is particularly noticeable in Cyclops fuscus (Fig. 1223). 



In the Cyclopidae the antennae are symmetrical and, in the male, 

 are modified to form grasping organs. In the Centropagidae it is 

 only the right antenna of the male that is so modified. 



In many of the species of Diaptomus the antepenultimate seg- 

 ment of the right antenna of the male has a distinctive form. 

 This may be a hyaline lamella extending the length of the segment, 

 or it may be an extension of one side of the segment in a process 

 which varies from a blunt projection to a hook or, in some cases, 

 a long, slender digitiform extension. The armature of this segment 

 is constant and is one of the important characteristics used in 

 distinguishing species. 



