ISOMETRINAE 191 



pinnule is P4, P5 or exceptionally Pg, of up to 15 segments and of the same length as, 

 or slightly longer than, the last oral pinnule. The genital pinnules extend to P13 in 

 smaller specimens, Pjg in larger, and the last of them may be of 18-26 segments and 

 9-14 mm. long. The first two segments of the genital pinnules are short and stout with 

 the aboral portions of their distal edges produced into spines. The segments carrying 

 the gonads are much more strongly expanded in the female than the male. In the 

 smallest specimen, which has arms 35 mm. long, none of the segments of the genital 

 pinnules is expanded. 



In the largest male the testes of the middle genital pinnules lie along the third to 

 eighth segments and they are expanded, more strongly on the aboral than the oral side 

 (Fig. 18 ^). The testes of the lower and outer genitals of big males, and those of younger 

 males, are smaller and consequently fewer segments are expanded. The third or the 

 fourth segment is the widest and longest. The remaining seg nents along which the 

 testes lie decrease in width gradually and evenly. The expanded segments are raised 

 into a keel-like crest in the mid-line. The narrow portion of the distal edge which is a 

 part of the crest may be thorny. The distal segments beyond the gonad are strongly 

 compressed. 



There are three females. In the smallest only the third to fifth segments of the 

 genital pinnules are expanded, as in /. vivipara ; the expansion appears to be a little less 

 on the aboral side than in /. vivipara. In the largest, a massive but broken specimen, 

 many of the genital pinnules (at least from P7-P10) have five segments, the third to 

 seventh, expanded ; in a few pinnules the eighth segment is slightly expanded (Fig. i8/?). 

 The expansion of the segments of one pinnule is not always even : not infrequently the 

 sixth segment is very narrow on the aboral side, the corner of the fifth being produced 

 alongside it, sometimes so much so as to meet the seventh. Other similar irregularities 

 occur. In the female, as in the male, the expanded segments are raised into a crest in 

 the mid-line. 



The ovary is a long fusiform body lying along the adoral side of the pinnule. It is 

 longer than the brood-pouch and the strongly expanded segments: in a pinnule in which 

 the brood-pouch lies along the third to sixth segments and they are strongly expanded, 

 the ovary extends to the eighth segment. In the largest female the brood-pouches are 

 empty. In one of medium size they are crowded with embryos and so large that they 

 bulge out beyond the edges of the expanded segments. A transverse depression runs 

 across the ventral face of the brood-pouch: it is due to a septum which divides the 

 pouch into two equal compartments. Its purpose appears to be to support the brood- 

 pouch for the contents of the two compartments are similar. One brood-pouch of the 

 medium-sized specimen was examined. The proximal compartment contained fourteen 

 embryos of which thirteen were young and without skeletal plates ; the fourteenth con- 

 tained skeletal plates. The distal compartment contained ten embryos without plates. 

 The brood-pouches of the third and smaller specimen are smaller: the proximal com- 

 partment of one contained four embryos in one of which there are plates ; the distal 

 compartment contained four embryos of which three have plates. 



