252 CRUSTACEA. 



The series of ontogenetic stages described above refers to the 

 Decapoda, but the Schizopoda (Euphausiidae} and the Stomatopoda 

 are in this respect closely related to this order. The Calyptopis 

 stage of the Euphausiidae might be assumed to be a Protozoaea 

 or a Zoaea stage if it did not differ from the latter in the absence 

 of the second pair of maxillipedes. In the metamorphosis of the 

 Stomatopoda, on the other hand, we see that a suppression of 

 the thoracic segments and the limbs belonging to them, similar to 

 that in the Decapodan Zoaea, leads to larval forms which have been 

 called the Pseudozoaeae of the Stomatopoda. 



A second series of forms among the Malacostraca, however, 

 including the Cumacea and the Arthrostraca, shows, in connection 

 with the care of the brood which there prevails, an entire dis- 

 appearance of metamorphosis. In these groups we have conditions 

 related to those found in the Mysidae and Leptostraca. We do, 

 indeed, find in the belated appearance of the last pairs of thoracic 

 limbs in the Isopoda a last remains of that ontogenetic tendency 

 which, in the Decapoda, led to the development of the Zoaea stage. 



8. Leptostraca. 



The Leptostraca (Nebalia), like the Mysidae, have no free- 

 swimming larval stage. When the young animals leave the shell- 

 cavity of the mother, which is used as a brood-chamber, they have 

 in all essential points (METSCHNIKOFF, No. 82) attained the final 

 shape. Metamorphosis is therefore here, as in the Mysidae, confined 

 to those stages which were passed through in the brood-cavity after 

 the shedding of the egg-integument. With regard to the appearance 

 of the different limbs, the order from before backward is retained. 

 In this respect, and in the absence of a distinctly marked Zoaea 

 stage, the Leptostraca nearly approach the Phyllopoda. The three 

 pairs of Nauplius limbs appear first. There then follows a stage 

 in which these three pairs of limbs have advanced in development, 

 while behind them can be recognised four other pairs (two pairs 

 of maxillae and the two anterior pairs of thoracic limbs). This 

 stage thus shows, in the number of limb-rudiments, a certain agree- 

 ment with the Zoaea form. At a later stage, the rudiment of a 

 third pair of thoracic limbs can be made out. The embryo lies in 

 the egg, curved in such a way that the ventral surface of the caudal 

 region is in contact with the ventral surface of the anterior part 

 of the body. The egg-integument then splits, and the larva, which 

 is thus freed, but is still enveloped in the larval cuticle, and which 



