270 CRUSTACEA. 



of a frontal spine found between these organs, as well as by that 

 of the thoracic limbs. The abdominal segments now increase greatly 

 in size, so that they are soon longer than those of the thorax. 

 The third pair of maxillipedes can now be recognised as a short 

 biramose limb (Fig. 123 B, III). Behind this, the rudiments of 

 the ambulatory limbs (IV- VIII) are just visible, and still slighter 

 prominences indicate the limb-rudiments on the five anterior 

 abdominal segments. It is thus evident that, in Penaeus, the limbs 

 have retained the order of development from before backwards. 

 An exception to this order, however, is afforded by the sixth pair 

 of pleopoda, which can at this stage already be recognised as bilobed 

 rudiments (a 6 ), thus preceding the other abdominal appendages in 

 development. This pair of pleopoda develops as the lateral portions 

 of the caudal fin (Fig. 123 C, a), while the small limb-rudiments on 

 the five anterior segments of the abdomen appear to be temporarily 

 suppressed during the later Zoaea stage (Fig. 123 (7, aj-a 5 ). In 

 such a later Zoaea stage, the five joints in the protopodite of the 

 first antenna seem also to have disappeared ; the rudiments of the 

 five pairs of ambulatory limbs can now be recognised as small 

 biramose appendages (IV-VIII). 



In the Mysis stage which follows (Fig. 123 D), the antennae have 

 ceased to function as locomotory organs, the locomotory function 

 being now performed by the biramose thoracic limbs. The cephalo- 

 thorax is now small as compared with the abdomen, which develops 

 greatly, and in which the pleopoda have now reappeared. Dorsal 

 spines appear on the abdominal segments from the second to the fifth. 

 The anterior antenna (1) now exhibits a three-jointed protopodite and 

 two still unjointed flagella; the latter become jointed at a future 

 period ; olfactory hairs develop, and an auditory organ forms at 

 the base of this limb. In the second antenna (#), the exopodite 

 has changed into a scale (squame) fringed with setae, while the 

 endopodite becomes transformed into the adult flagellum. The 

 upper lip has lost its spinous process. On the mandible, the two- 

 jointed palp of the adult develops. In the anterior maxilla, the 

 endopodite degenerates into a short truncated limb, while the 

 exopodite is completely lost. The exopodite of the second maxilla 

 changes into the large respiratory plate. The first maxillipede 

 resembles that of Sergestes, the protopodite growing out into a large 

 cutting plate, while the endopodite and exopodite are retained as 

 small appendages, and a branchial pouch sprouts from the basal 

 joint (Fig 91 a, i>. 194). The second and third maxillipedes (//, III), 



