284 



CRUSTACEA. 



laticaudes," in wliicli the abdomen arises with a broad base as the direct pro- 

 longation of the thorax. Richtees, on tlie other hand, has conjectnrally 

 referred the Brcvicaudes to Ihacus and Parihacus. Fossil Phyllosonia have 

 been recognised in^the Solenhofen slates. 



F. Thalassinidea. 



The larvae of the Thalassinidea {Gehia, Calocaris, Callianassa, 

 Calliaxis), known to us from the treatises of G. O. Sars (No. 149) 

 and Glaus (Nos. 6, 7, and 8), are allied to the Caridid larvae by 

 the shape of the body, the possession of a long rostral spine, and 



the peculiar armature of the 

 abdomen (wanting in Gehia) ; 

 this latter consists of a long 

 dorsal spine on the second 

 and shorter spines on the; 

 three following abdominal 

 segments. Their metamor- 

 phosis is, however, of special 

 interest on account of tlie 

 complete transition which 

 tliey exhibit between the 

 manner of development of 

 the Garidids and that of the 

 Anomura and Brachyura. 



The youngest larval stage 

 of Gehia (Fig. 132 yl) is a 

 Zoaea which is distinguished 

 from the Zoaeae of other 

 ]\Iacrura chiefly by the fact 

 that only the two anterior 

 maxillipedes (m.f, mf") func- 

 tion as biraraose swimming 

 limbs, while the third maxil- 

 lipedes, as well as the four 

 following ambulatory limbs, 

 are found as rudiments devoid of setae, the four anterior pairs being 

 biramose, while the most posterior pair is still simple. The fifth 

 pair of ambulatory limbs, as well as the pleopoda, are still altogether 

 wanting. The telson, Avhich is not yet marked off from the sixtli 

 abdominal segment, is a spade-shaped plate somewhat indented at 

 its posterior edge and beset with setae. The paired eyes, which 

 are still immovable, recall those of the Anomuran larva, in the 



Fio. 13-2. — Two larval stages of Gehia Uttnralls (aftiT 

 G. O. Sars). A, Zoaea stage (dorsal view). /.', 

 Mysis stage (side view), a', first antenna ; n", 

 second antenna; a'^-a^, rudiments of the five 

 posterior abdominal appendages; mf, mf", mf"', 

 the three maxillipedes ; p^-p^, the five ambula- 

 tory limbs ; oc, Nauplius eye. 



