330 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



plankton. The original examination of material 

 obtained from rearing exi^eriments did not dis- 

 close these structures, but a photomicrograph (fig. 

 9) shows them clearly. With this proof of their 

 existence and their subsequent discovery on the 

 planktonic specimens, the presence of frontal 

 organs was established. 



The first antenna now bears 3 setae on the an- 

 terolateral and 2 on the posterolateral surfaces, 

 as well as 3 terminal setae. This is a total of 8 

 setae as compared with the 5 found on this ap- 

 pendage in the preceding stage. The segmenta- 

 tion remains the same as in the fourth nauplius. 



The endopod of the second antenna is unseg- 

 mented and bears 3 lateral and 4 terminal setae. 

 Two of the lateral setae originate from the same 

 point close to the distal end of the endopod. The 

 exopod of the appendage now bears 9 setae, with 

 the addition of a short lateral seta which is the 

 most proximal to the body. 



Figure ».— Photouiicrograph of fifth nauplius. (Note 

 frontal organs.) 



The basal portion of the protopod of the man- 

 dible has become swollen and knoblike and bears 

 a ring of toothlike structures. The endopod has 

 become transparent in some specimens due to loss 

 of its musculature. The setation is the same as 

 in previous stages. 



The maxillae and maxillipeds are further de- 

 veloped, and cover the major portion of the ven- 

 tral surface of the body posterior to the labrum. 

 They remain nonfunctional. The developing max- 

 illae and maxillipeds are biramous. The exo- 

 pods of the maxillae take the form of swollen 

 knobs protruding from the distal portions of the 

 protopods and have been termed scaphognathites. 

 The exopods of the maxillipeds are palplike in 

 form. 



In dorsal view the body appears to be in two 

 segments. Tliis is due to the outline of the pos- 

 terior edge of the developing carapace, which can 

 be seen under the cuticle at about the midpoint of 

 the body. 



FIRST PROTOZOEA 



Forty-two first protozoea (figs. 11 and 12) of 

 P. dmoranmn raised from eggs measured from 

 0.86 to 1.02 mm. in body length, from 0.35 to 0.44 

 mm. in body width at the widest part of the cara- 

 pace, and from 0.40 tx> 0.49 mm. in carapace 

 length. Seven first protozoea taken from the 

 plankton measured from 0.98 to 1.14 mm. in body 

 length. 



The first protozoea represents the most radical 

 change in the form of the larva up to this point. 

 The body is clearly divided into two parts. The 

 anterior part is covered by the carapace, which 

 is just under one-half the body length. The cara- 

 pace is rounded anteriorly with a notch at the 

 midline. The posterior edge of the carapace is 

 nearly straight and covers the basal portion of 

 the second maxillipeds. The narrower posterior 

 part is divided into a thorax of six segments and 

 an unsegmented abdomen. The junction of thorax 

 and abdomen is marked by a slight swelling in the 

 latter. 



Another feature differentiating this stage from 

 the preceding one is the development of compound 

 eyes. These can be seen under the carapace, but 

 do not become stalked imtil the next stage. A 2- 

 lobed labium just posterior and dorsal to the la- 

 brum and a digestive tract are other structures 



