272 



CRUSTACEA. 



in the Caridea, but are merged in the general embryonic develop- 

 ment which takes place in the egg. The embryo leaves the egg, 

 as a rule, as a peculiarly-shaped Zoaea (Fig. 124), which in certain 

 characters anticipates the Mysis stage, and thus in many respects 

 is more advanced than the typical Zoaea. We may take as an 

 example a larva described by GLAUS (No. 113), and referred to the 

 ontogeny of Hippolyte. In this larva we can distinguish an anterior 

 cephalo-thoracic and a posterior abdominal region. The latter is long 



and already shows its full number 

 of segments (in forms related to this 

 Zoaea, however, the separation of 

 the sixth segment from the telson 

 is often at first indistinct). The 

 telson is, as a rule, no longer cleft 

 (furcal structure), but has the form 

 of a broad plate with a spinous 

 posterior margin ; in individual 

 cases, however (e.g., Pontophilus. 

 G. 0. SARS, No. 151), a furcate 

 condition with two lateral wings is 

 found. The dorsal shield carries a 

 simple pointed rostrum and short 

 supra-orbital and antennal spines, 

 but there is no further development 

 of spinous processes. In many 

 larvae of this group, however, there 

 is a very prominent dorsal spine on 

 the second abdominal segment, as 

 well as smaller ones on the three 

 following segments. In other re- 

 spects, the spinous armature of this 

 region of the body undergoes great 

 variation in the forms belonging 

 to this group. Near the Nauplius eye, the paired eyes are distinct 

 and already stalked. 



In the anterior region of the body, besides the seven pairs of 

 limbs occurring in the Zoaea, the eighth pair (third maxillipedes, 

 Fig. 124, Mf") appear in the form of well developed swimming 

 limbs. The region following this, which comprises the five segments 

 carrying ambulatory limbs, is almost unrepresented, its segments and 

 limbs only becoming apparent at a later period. The pleopoda are 



Fio. 124. Zoaea of Hippolyte (after GLAUS, 

 from BALFOUR'S Text-book). MX', first, 

 MX", second maxilla ; Mfi, Mf", M/m, 

 first, second, and third maxillipedes. 



