LEPTOSTRACA, SCHIZOPODA, DECAPODA. 



157 



becomes the rudiment of the labrum, behind which the stomodaeal invagination 

 soon appears. The latter, as a rule, lies in the space between the first and second 

 pairs of antennae, but Kingsley has specially emphasised the post-oral position 

 of the first pair of antennae in Crangon. Paired swellings arise near the 

 posterior margin of the oral aperture ; these are the rudiments of the paragnatha 

 or bilobed lower lip. 



The later stages (Figs. 80, 81, and 83) are characterised by the 

 further growth of the thoraco-abdominal rudiment, which soon breaks 

 up into segments, and also by the development of the posterior pairs 

 of limbs which appear from before backward. At the same time 

 the typical biramose character is found in the anterior limbs which 

 were the first 

 to develop, as 

 well as a seg- 

 mentation cor- 

 respondingwith 

 the developing 

 joints (for the 

 number and 

 shape of the 

 limbs of the 

 hatching em- 

 bryo, which 

 vary greatly in 

 the different 

 groups of the 

 Decapoda, see 

 below, p. 257 : 

 Metamorphosis 

 of the Deca- 

 poda). The 

 optic lobes gain 

 in independ- 

 ence, bulging 

 forwards and 



gradually rising from the surface below them, so that the club-like 

 shape of the stalked eye can be recognised in the rudiment (Fig. 81). 



The posterior part of the body undergoes important alterations. 

 There is here an early separation of the terminal or anal segment 

 (Figs. 80, 81, T) from a budding zone composed of large cells which 

 lies in front of this segment (in the embryo, however, owing to the 

 forward curvature of the tail, further back). The anal aperture is 



TTIX, 



Fig. 80. — Embryo of Astacus fluviatilis, with the thoracic feet 

 beginning to form (after Reichenbach, from Lang's Text-book). 

 A, eyes; a lf a,, first and second antennae; ab, abdomen; g, rudi- 

 ment of the brain (procerebrum+antennal ganglion) ; go, optic 

 ganglion; /-, upper lip; m, mandible; mx t , mx. 2t first and second 

 maxillae; T, telson ; t 7 , t 8 , thoracic limbs; t lt t n , maxillipedes ; 

 t 4 , t 8 , ambulatory limbs ; ts, rudiment of the thoracic shield. 



