V CRUSTACEA—OXTOGEXY 393 



becomes differentiated from before backward. But here also the last pair of pleopoda 

 advances more rapidly than the others. 



Laxval history of the Stomatopoda (Fig. 265). 



Unfortunately we do not know the whole series of larval forms in any of the 

 Stomatopoda. They belong to two types, one of which is called the Urichthus, the 

 other the Aliina type. "We shall only consider the first. 



A. Youngest known Eriohthoid larva {A). — Three regions can be distinguished 

 in the body, an anterior, a middle, and a posterior. The anterior corresponds 

 with the head, and from it arises as a fold of the integimrent a large dorsal shield 

 which covers the second region as well. The head carries besides a median eye the 

 two large stalked eyes, the two pairs of antennse, the pair of mandibles and the two 

 pairs of maxUlfe. The second region consists of 5 segments, corresponding with the 

 5 anterior thoracic segments, and carries 5 pairs of biramose swimmerets (/- V), the 

 last 3 of which decrease in size from before backward. The 5 pau'S of limbs answer 

 to the 5 pairs of oral limbs in the adult Stom.ato2yocla. The third region consists of 3 

 short limbless segments (also covered by the dorsal shield) corresponding with the 3 

 posterior thoracic segments, and a very large caudal plate, also devoid of appendages. 



E. In a somewhat older 2d larva a new segment (the most anterior abdominal 

 segment) with one pair of limbs has formed in front of the caudal plate. The 2d 

 pair of thoracic feet shows alterations preparatory to its transformation into large 

 seizing feet. 



C. In a third larva (B) 2 new segments mth the rudiments of their limbs, and 

 in older larvae all the abdominal segments with their pairs of limbs excepting the 6th, 

 have begun to form in front of the caudal plate, while the 3 posterior thoracic 

 segments are still limbless. 



D. In a fourth Eriohthoid larva the 2 anterior pairs of thoracic feet have lost 

 their exopodites, but on the other hand the rudiments of epipodites (gills) appear on 

 them. The three subsequent pairs of thoracic feet are reduced, and the last three 

 thoracic segments are still limbless. On the 6th abdominal segment the rudiments 

 of the limbs (uropoda) appear. 



E. In the subsequent stages, the 3rd, 4th and 5th pairs of thoracic limbs are 

 cotapletely reduced, or else are represented only by small sac-like prominences ( C). 



F. There now follows, after various preparatory intermediate stages, the 

 completely developed Erichthus larva. (D) The 3d, 4th, and 5th pairs of thoracic 

 limbs again appear in their definitive form, so that now the 5 anterior trunk limbs 

 are developed as seizing or oral limbs. On the last three thoracic segments the 

 rudiments of the biramose ambulatory limbs appear. By this time, not only the 

 full number of segments, but the full number of limbs of the adult animal is reached. 



In this larval history of the Stonuctopoda we note 



(1) That the segments of the body become differentiated from before backward. 



(2) That the limbs also as a general rule follow the same order. We observe 

 this principally in the case of the abdominal limbs, since in the youngest Erichtlioid 

 larva the 5 anterior thoracic limbs are already developed. Of the pleopoda, the last 

 (uropoda) appeared last, in opposition to the Decapoda, in which this latter takes 

 precedence of all the others and even of some of the thoracic limbs. 



3. The last three weakly developed thoracic limbs (ambulatory limbs) form the 

 chief exception to the rule stated under (2), in that they first appear after the 

 pleopoda. 



4. It is a striking fact that the 3d, 4th, and 5th pairs of trunk limbs which are 

 early developed as biramose rowing feet become completely reduced, and are then 

 again formed in their definitive shape in the oldest Erichthus stage. 



