76 THE CRUSTACEA 



of the free-living Eucopepoda they assist in swimming. They are 

 further modified in many cases, in the male sex, to act as prehensile 

 organs for seizing the female. They are most fully developed in 

 the Gymnoplea, where they may exceed the body in length and 

 may consist of twenty-five segments (Fig. 41, A, a'). Throughout 

 the Gymnoplea and also in the Cyclopidae and Asterocheridae, 

 where the number of segments is less, Clans and Giesbrecht have 

 demonstrated their homology with those of the twenty -five- 

 segmented form. As an example we may give Giesbrecht's com- 

 parison of the segmentation of the antennules in a species of 

 Cyclops, with the typical arrangement as found in many Gymnoplea. 



Gymnoplea 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25 

 Cycbpst 1, 2, 3,4,5,6,7,8, 9,10,11,12,13, 14^ 16, 16, 17~~ 



Cyclops? 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 127l3, 14, 15, 16, 17 



In the Harpacticidae and other families with a reduced number 

 of segments the homology has not been demonstrated, but there 

 can be little doubt that the twenty-five segments of the Gymnoplea 

 represent the typical and primitive arrangement from which the 

 others have been derived. In the parasitic forms the antennules 

 are usually greatly reduced. In the Caligidae the basal portions 

 of the antennules become coalesced with the front of the head, form- 

 ing a bilobed prominence. In some genera this bears, on either 

 side, a sucker which serves to attach the parasite to its host. Ex- 

 cept in this instance, however, the antennules do not become con- 

 verted into organs of attachment as the antennae frequently do. 



In most, perhaps in all cases, the antennule, as in other Crustacea, 

 bears sensory setae of the type to which Giesbrecht has given the 

 name " aesthetascs," and these are commonly more numerous in the 

 male sex. In some cases each antennule bears only a single 

 aesthetasc, which may then be of relatively great size. 



In those Eucopepoda in which the antennule of the male is 

 transformed into a clasping organ for seizing the female, the distal 

 is flexed upon the proximal portion (Fig. 43, A). It is probable 

 that the point at which this flexion takes place is the same in all 

 cases, corresponding to the articulation between the eighteenth 

 and nineteenth of the primitive series of segments. The proximal 

 portion of the appendage becomes more or less swollen, owing to 

 the strong development of flexor and extensor muscles of this joint, 

 and the opposed edges are often armed with teeth or spines. In 

 those Gymnoplea in which this clasping apparatus is developed, 

 only one, generally the right, antennule is modified (Heterarth- 

 randria). In the Podoplea, on the other hand, the modification, 

 when present, is bilateral. The occurrence of this modification of 

 the male antennules in many families of Eucopepoda which, in any 



