MORPHOLOGY OF CYCLOPS. 11 



(3) a short basal flexor, originating in base of first joint just distal to last muscle and 

 inserted on flexor side of second joint. 



2. Muscles acting on 9th-13th joints : a fine muscle extending from tbe insertion of 

 basal extensors to the flexor side of 9th joint ; part at least being contimied to the base of 

 13 on its extensor side must be an extensor of the 13th joint. A pair of slips from the 

 flexor edges of 2 and 3 inserted on the flexor side of 9. 



3. Flexor of the clasping-joint : a great thick-bellied muscle, taking origin in 9 and 

 inserted in apodeme of 15, which acts, indeed, as a tendon by its ventral insertion lying 

 inside 14 (which is exceptional, for, as a rule, the proximal segment of an appendage 

 overlaps the distal). 



4. Plexor of last piece : origin on extensor side of 16, insertion into flexor proximal 

 edge of 17. 



The noteworthy points are these : the 15th segment acts purely as a sesamoid in the 

 flexor of the 15tli segment, changing the direction to make the pull more direct : in 

 flexion it lies almost entirely inside 14. An extraordinary mechanical device is the " lock- 

 spring" arrangement for extension of the IGth and i7tli segments, and possibly of tlie 

 13th. Both the 14th and 16th segments have on their flexor edges short processes bearing 

 a horizontal medifixed spine like the ' pili mcdjnr/heacearum ' of botanists. The ends of 

 the spines lie on the two adjoining segments in each case, which, indeed, are tumid and 

 convex (the 13th only slightly so), so that in flexion the spines are forced over them, 

 and when the muscles relax their elasticity determines extension. The hooked and ser- 

 rated spines on the llth-14th segments must help to tighten the hold on the female. 



This pair of appendages is, in both sexes, acted on as a whole by extrinsic muscles, 

 inserted into the proximal end of its basal joint, running obliquely upwards and backwards 

 to the sides of the carapace. These are a double extensor, nearly vertical, pulling the 

 antennule forward, a double shoi'i flexor taking origin just a little further back, and a 

 double long flexor running obliquely upwards and backwards internal to the other extrinsic 

 muscles of the cephalic appendages, to be inserted at the upper lateral part of the cara- 

 pace in the region of the maxillipeds. (This muscle consists distinctly of two great 

 fascicles, one in front of the other, but I cannot make out distinct insertions.) This might 

 be regarded as lending some support to the view that the carapace is formed in part by 

 a backward extension of the cephalic region ; but mechanical advantage may have 

 determined the gradual backward shifting of the insertion. This pair of appendages is 

 innervated from the postero-lateral region of the brain. 



II. Antenna projjer (PL I. fig. 3), clinging antenna or second antenna: 4-jointcd, 

 flexor, postaxial or outer or hinder edge serrated with fine teeth along its whole length. 



Basal joint oblong, concave externally, convex internally, bears at distal end on 

 extensor side two flexible hairs sparsely setose; on flexor side, on a distinct bat slight 

 prominence, a long flexible hair as long as rest of limb, fringed abundantly with coarse 

 almost tooth-like setae, probably equivalent to the exopodite or palp found here in some 

 other families of the Copepoda. 



Second joint shortest of all, hinged to allow limited extension and flexion ; cylindrical, 

 bears a thinly fringed plume halfway up extensor edge. 



2* 



