THE COPEPODA 81 



of, at most, seven (perhaps eight) segments, but the number is 

 often much reduced, and, as in the case of the maxilla, the limb 

 is commonly modified into a clinging organ with a strong terminal 

 claw, or, as in a section of the Harpacticidae, into a subchelate, 

 prehensile " hand." 



The five following pairs of appendages (the thoracic limbs of 

 the ordinary terminology, the second to the sixth of the system 

 here adopted) are in some Gymnoplea all similar and in the form 

 of biramous swimming-legs. This form is retained by some, at 

 least, of these limbs in all Copepoda in the later larval if not in the 

 adult stage, and constitutes on A of the most general characters of 

 the sub-class. 



Each consists typically of a broad and flattened protopodite of 

 two segments, and of an endopodite and exopodite, each with, at 

 most, three segments, flattened, and bearing marginal natatory setae 

 together with, on the outer edges, strong spines (Fig. 3, B, p. 8). 

 The proximal segments of the 'protopodites of each pair are con- 

 nected with each other across the middle line by a plate formed 

 by a transverse fold of the sternal integument (the " Bauchwirbel " 

 of Zenker), so that in the backward and forward movement in 

 swimming the two appendages move as one. 



The last (sixth) pair of thoracic limbs are similar to the preceding 

 pairs only in the females of some genera of Gymnoplea. In the 

 male sex of that order they are always modified into copulatory 

 organs, often very complex, by means of which the spermatophores 

 are affixed to the copulatory aperture of the female. This modifica- 

 tion is asymmetrical on the two sides in correlation with the 

 asymmetrical development of the internal generative organs in 

 the group (Fig. 43, B). In the females of many Gymnoplea the 

 appendages exhibit every stage of reduction even to complete dis- 

 appearance. In the Podoplea the appendages are always present 

 (Fig. 44, A, vi.), except in the more degraded parasites ; always 

 vestigial, consisting of one or two small segments ; and are not 

 specially modified in the male. In some Harpacticidae and Ascidi- 

 colidae, however, they become enlarged in the female sex into 

 plate-like appendages serving to protect the egg-masses. It is 

 noteworthy that these vestigial limbs of the last pair may persist 

 even in cases where the preceding pair of limbs is suppressed. 



In the Podoplea (but not in Gymnoplea) the genital apertures 

 of the female on the first abdominal somite are guarded by valvular 

 plates moved by muscles. These valves have been supposed to 

 represent a vestigial pair of appendages. 



In the parasitic forms, with the loss of the power of locomotion, 

 the thoracic limbs become more or less reduced. In Lernaeocera, 

 for example, they persist as microscopic though completely formed 

 limbs set at long intervals along the length of the unsegmented 



6 



