OSTEACODA. 



207 



The Nanplius stage {first free larval stage, Fig. 99 A) is, as already mentioned, 

 distinguished by the presence of a bivalve shell iirovided with an adductor 

 muscle (SM). At its highest point is found the Nauplius eye, which is here 

 very large and provided with paired lenses. The alimentary canal, which consists 

 of oesophagus, mid-gut, and hind-gut, is still without hepatic appendages. The 

 oral aperture is overarched by a large upper lip {OL). The three pairs of limbs 

 differ somewliat in shape from those of the Phyllopodan Naujjlius, being here 

 developed as uniramose crawling limbs. The first tw(i pairs already approach 

 the adult foi'm. The first antenna {A') is bent inwards, and consists of four 

 joints, tlie three terminal joints being beset on their upper surfaces with rowing 

 setae. Tlie second antenna {A") ah-eady shows the characteristic knee-like bend 

 inwards. The mandible {3fd) has still the form of a three-jointed crawling 

 limb, its tip ending in a large hooked seta. The future masticatory blade is at 

 this stage merely a small serrated process of the basal joint. 



In the second larval stage (Fig. 99 B] whicli follows, the mandible loses the 

 primitive character just described. Besides this, behind the mandible, two more 



fiM^I 



Fig. 101.— Sixtli larval stage of Cypris (after Clads, from Balfour's Text-hook). Mx', tirst, 

 Mx'i, anil /', second maxilla ; /", /'", first and second pairs of feet ; Fu, caudal fork ; L, liver ; 

 SD, shell-gland. 



rudiments have arisen, those of the first maxilla {Mx) and of the first pair of 

 feet (/"). The body has correspondingly lengthened. The basal joint of the 

 third limb is now transformed into a large triangular masticatory blade 

 (mandible) toothed on ics inner edge, the rest of the limb persisting as a four- 

 jointed mandibular palp. Attached to the basal joint of the latter can be 

 I'ecognised the first rudiment of a hand-shaped aj^pendage beset with setae 

 (exopodite). The next rudiment (that of the first maxilla) is a leaf-shaped 

 curved plate {Mx'), while the first ambulatory limb (/") is an unjointed climbing 

 organ provided with a terminal hook. 



In the third stage, the maxilla approaches its adult form, two strong, toothed, 

 masticatory blades and the rudiment of a third being discernible. The rudi- 

 ment of the movable gill-plate (exopodite) beset with hairs can also now be 

 recognised. In the following fourth larval stage (Fig. 100 A) there is uo 

 essential alteration in the antennae and the mandibular palp beyond a richer 

 provision of setae ; on the other liand, on the first maxilla {Mx') four masticatory 



