CRUSTACEA. 



485 



(the sixth hardly differentiated), each with a pair of appendages (the two 

 hindermost still rudimentary) which have become gradually developed from 

 before backwards. The three hindermost thoracic segments are still without 

 appendages. 



Some changes of importance have occurred in the other parts. Both 

 antennae have acquired a second flagellum, but the mandible is still without 



FIG. 227. ADVANCED ERICHTHUS LARVA OF SQUILLA WITH FIVE PAIRS OF 



ABDOMINAL APPENDAGES. (From ClaUS.) 



f, first maxilliped ; g. second maxilliped. 



a palp. The first and second pair of maxillipeds have both undergone 

 important modifications. Their outer ramus (exopodite) has been thrown 

 off, and a gill-plate (epipodite) has appeared as an outgrowth from their 

 basal joint. Each of them is composed of six joints. The three following 

 biramous appendages have retained their earlier characters but have become 

 much reduced in size. In the subsequent moults the most remarkable new 

 features concern the three posterior maxillipeds, which undergo atrophy, and 

 are either completely lost or reduced to mere unjointed sacks (fig. 228). In 



FIG. 228. ADVANCED ERICHTHUS LARVA OF SQUILLA WHEN THE THREE 

 POSTERIOR MAXILLIPEDS HAVE BECOME REDUCED TO MINUTE POUCHES. 



(From Clans.) 



the stage where the complete Erichthus type has been reached, these 

 three appendages have again sprouted forth in their permanent form and 

 each of them is provided with a gill-sack on its coxal joint. Behind them 

 the three ambulatory appendages of the thorax have also appeared, first 

 as simple buds, which subsequently however become biramous. On their 

 development the full number of adult appendages is acquired. 



The most noteworthy points in the developmental history detailed above 

 are the following : 



(i) The thoracic and abdominal segments (apart from their appendages) 

 develop successively from before backwards. 



