198 



CRUSTACEA. 



seventh segment, a non-lobate rudiment is found, and behind this, 

 the rudiments of two or three more segments. Behind the first 

 pair of maxillae, the rudiment of a second pair can be distinguished 

 as a transverse prominence. The shell-gland is distinctly visible 

 in the lateral part of the dorsal shield, which has now slightly 

 increased in size. At this stage the heart can already be made out, 



FIG. 93. Three larval stages of Apus cancriformis (after CLAUS, from LANG'S Text-book). 

 A, larva just hatched (Metanauplius). B, second larval stage with the anterior maxillae 

 and seven (or eight) thoracic segments. C, fourth larval stage with about fifteen thoracic 

 segments. 1, first, 2, second antenna ; 8, mandible ; , first maxilla ; I.-XIII. , first 

 thirteen trunk-segments ; fs, frontal sensory organ ; s, dorsal shield ; L, liver. 



reaching from the maxillary region posteriorly to the sixth thoracic 

 segment. As the segments develop posteriorly, new chambers are 

 added to the heart (Fig. 87, p. 178). 



In the fourth larval stage (Fig. 93 0), further rudiments of limbs 

 are seen in the act of forming on the thoracic segments ; on the 

 seven anterior segments these are distinctly lobate, on the next two, 



