SERGESTIDAE. 261 



the lateral portions of the caudal fin (sixth pair of pleopoda) have 

 now appeared. 



The next moult of the larva produces a very decided change in 

 the external shape of the body. The larva now passes into the 

 so-called Mysis or ScMzojwd-like stage (Fig. 120 .4), which was 

 described by Dana under the name of Sceletina armata. The 

 antennae have lost their locomotory functions and tend to assume 

 their adult form ; the paired eyes (o) have become movable and 

 stalked, and the Nauplius eye (oc) is still retained. The seven pairs 

 of biramose swimming limbs (viz., the three pairs of maxillipedes 

 and the four anterior pairs of ambulatory limbs) .function as loco- 

 motory organs. The larva, from this time onward, ceases to progress 

 in a jerking manner, as in the preceding stage, and swims evenly and 

 swiftly. The dorsal shield has still the same general shape, but, in 

 its anterior part, the stalked eyes have become cut off from it. The 

 indentation of the shield which contains these latter is marked by 

 a pair of antero-lateral spines, while the spines at the posterior 

 margin of the shield have disappeared. The cephalo-thoracic shield 

 remains short in comparison with the abdomen which, in the next 

 stages, increases still more in length. In the abdomen, we can 

 recognise six perfectly distinct segments and the caudal fin, consist- 

 ing of the telson and the greatly enlarged sixth pair of pleopoda (ab 6 ). 

 The first antenna (a') now consists of a two- (later three-) jointed 

 protopodite and a short terminal joint which is to be considered as the 

 representative of the nagellum and is richly provided with feathered 

 setae. The second antenna (a") is a reduced biramose limb almost 

 without setae. In the later Mysis stages this appendage makes a 

 fresh growth, its endopodite being transformed into the flagellum 

 and its exopodite into the scale -like appendage. The mandibles 

 (md) are simple masticatory blades without palps. The maxillae 

 (m$c\ mx") have essentially the same form as in the preceding stages. 

 This is also the case with the first pair of maxillipedes (mf), the 

 segmentation of which is somewhat less distinct than in the pre- 

 ceding stages. The six pairs of swimming limbs which follow 

 (second and third maxillipedes and first four pairs of ambulatory 

 limbs) are biramose and very similar in form. Each limb consists 

 of a two-jointed protopodite, a long, four-jointed endopodite, and a 

 shorter exopodite Avhich i?, however, indistinctly divided into 

 numerous joints. An ample provision of setae enables these limbs 

 to act as powerful swimming organs. 



Later Mysis stages (Fig. 120 B\ which are principally distin- 



