384 COMPARATIVE ANATOMY CHAP. 



larva only by the fact that in the posterior third of the larval body, in front of its 

 posterior end, the rudiments of the 5 anterior trunk segments (/- V} and their limbs 

 can be recognised under the integument. 



2d Larval Stage. The Nauplius casts, its skin and the larva in the 2d stage 

 (Fig. 257, B] shows considerable modifications. The anterior body is broadened, the 

 posterior body elongated in the shape of a cone. There are two frontal projecting 

 stylets (frontal sensory organs). The dorsal shield has increased in size. On the 

 basal joint of the 3d pair of limbs (mandibular limbs) a masticatory process has 

 formed. Behind the mandibular limbs are the rudiments of the first pair of maxillae 

 (4). The 5 anterior segments of the trunk can be more clearly distinguished, and 

 also the rudiments of the 3d and 4th pair of trunk limbs, the latter as transverse 

 ridges. Later a 6th segment arises, and behind it the rudiments of the 2 subsequent 

 segments. The bulgings at each side of the anus have become elongated into con- 

 siderable furcal processes. 



3d Larval Stage. This is entered on at the 2d moult. There are 6 anterior 

 pairs of lobate trunk limbs, whose size and degree of differentiation decreases from 

 before backward ; these already clearly show the characteristic form of the Phyllo- 

 podan swimmerets, with their endites, exopodites, and branchial sacs. Behind the 6 

 anterior trunk segments 2 more, and later 3 more, can be distinguished, and behind 

 the anterior maxillae the posterior have begun to form. The dorsal shield at first 

 covers only the two anterior trunk segments. 



The 4th Larval Stage appears with 7 anterior pairs of lobate trunk limbs (Fig. 

 257, C], Three to four anterior segments are covered by the dorsal shield. The 8th 

 and 9th pairs of limbs show the beginning of lobate formations ; the 10th to 13th 

 pairs of trunk limbs just arising. Rudiments of the paired eyes are visible. 

 The rowing antennae (2d antennae) are provided with large jaw hooks. 



5th Larval Stage. The 9 anterior pairs of trunk feet are lobate, the 10th is in 

 the act of forming a lobe, the llth, 12th, 13th, and 14th pairs of feet are forming. 

 Behind these are to be seen the rudiments of 6 new segments. Locomotion, 

 hitherto caused by the 2 anterior pairs of limbs, is now chiefly produced by the trunk 

 feet (swimmerets). 



The mandibular foot is, as compared with the earlier stage, very much 

 reduced, its principal part now being the masticatory process. 



Further Larval Stages. Frequent moults follow. New swimmerets are con- 

 tinually formed behind those already developed, and become differentiated from before 

 backward. The mandibular foot is more and more reduced, till nothing remains but 

 the masticatory process. The rowing limbs of the larvae (2d antennae) also become 

 reduced. The dorsal shield continues to widen. The form of the adult animal 

 develops very gradually. 



From this larval history we see, 1st, that the body and its appendages in general 

 become differentiated quite gradually from before backward, that new segments and 

 limbs progressively, though also occasionally irregularly, form behind those already 

 developed, and that these differentiations originate in the most posterior region of 

 the body. 



2d. That there occur deviations in details from this manner of formation. The 

 maxillae are an instance of belated appearance in the order of the limbs from before 

 backward. This fact is of importance because in the adult Phyllopoda as compared 

 with other Crustaceans the maxillae are very much simplified. 



3d. The mandibles, which in the adults are masticatory ridges without feelers, 

 are in young larvae well developed biramose limbs. 



4th. The posterior antennae, which are reduced in the adult, are, as large biramose 

 rowing arms, the chief organs of locomotion in the young larva. 



