DEVELOPMENT OF PLEOPODS IN M. SUBRUGOSA 223 



pleopods of the third segment (Fig. 5/) remain as before, as do those of the preceding 

 segment (Fig. pn). 



The next advance in development shov^^s the pleopods of the fifth segment approach- 

 ing the final form (Fig. 6a). All sign of the protopodal lobe has disappeared, the whole 

 limb is stouter and more elongated and supports more egg-carrying hairs. The third and 

 fourth pleopods still show a slightly lobose protopodite, and although the egg-carrying 

 hairs are now abundant, a plumose seta still remains on the fourth pleopod which is 

 figured (Fig. 6b). The pleopods of the second abdominal segment (Fig. 6d) are now 

 stiff'er and are beginning to carry hairs. Finally the adult form is realized (Figs. 6e-h) 

 and only growth, strengthening of the limb and greater proliferation of the ovigenous 

 hairs are required. 



In the male the pleopods of the first and second segments are modified to act as 

 copulatory appendages. In the late larval and early post-larval stages no appendage is to 

 be found on the first abdominal segment in either sex. In the male, however, with the 

 commencement of the modification of the second pleopods to form copulatory append- 

 ages, the supplementary copulatory appendages on the first segment make their appear- 

 ance. The appendages of the second segment are present in the early post-larval stages 

 as biramous pleopods (Fig. 4^), similar to those of the succeeding segments, and later 

 they degenerate to a protopodite with two projections representing the endopodite and 

 exopodite as already described (Fig. ^h). In the female, the exopodite disappears and 

 the endopodite persists in very simple form. The degenerate exopodite, however, 

 persists in the male, whilst the endopodite takes on a considerably modified form. 



The form of the second pleopod shown in Fig. 4/z is followed, in the male, by that 

 depicted in Fig. jn, and it consists of a simple protopodite bearing a simple styliform 

 endopodite and a very small bud representing the exopodite. Fig. jc shows further 

 development; an increase in size has taken place; especially of the endopodite, whose 

 distal half is now flattened and carries a border of short stiflF hairs. A keel runs along the 

 proximal half, and the flattened distal part appears to have been twisted on this portion. 

 Development continues by the expansion of the flattened spatulate tip of the endopodite 

 and the thickening and strengthening of the proximal stalk-like part. The exopodite 

 increases in size and projects as a very simple, single segment. Fig. ye shows what is 

 almost the mature adult form. The whole limb is very much larger. The endopodite is 

 large and expanded, and is bordered by short, stiff hairs, with a bunch of these covering 

 one corner. The exopodite is rigid and acts as a stop to the free movement of the endo- 

 podite in a lateral direction. 



The appendage of the first abdominal segment of the male first appears as a very 

 simple limb of two segments (Fig. yb), the proximal being long and curved with a small 

 terminal segment. It is this terminal segment which undergoes the most modification 

 before the limb achieves its final form. Fig. yd shows it lengthening and becoming 

 slightly turned at the tip; one or two hairs have made their appearance. The tip then 

 expands with one side curved and one side straight, giving a prow-like termination to 

 the limb (Fig. 7/). In the final form (Fig. yh), the protopodite is curved and strong and 



