NO. 10 CRUSTACEAN METAMORrilOSES — SNODGRASS II 



Our chief interest in the nauplius is the question of its theoretical 

 value in phylogeny. The naupHus has been Hkened to the polychaete 

 trochophore, and has been regarded as representing a primitive an- 

 cestral form of the Crustacea. The trochophore, however, is entirely 

 unsegmented and does not have the internal organization of the 

 nauplius. Later it becomes segmented by a direct division of the 

 posterior part of its body into a few primary somites. Likewise the 

 very young trilobite, known as a protaspis, at first shows no sign 

 of segmentation, but it soon becomes marked by transverse grooves 

 that divide it into a few primary segments corresponding with the 

 segments in the prosoma of the adult. A similar early direct segmenta- 

 tion occurs also in the ontogeny of the Xiphosurida. The nauplius, 

 therefore, would appear to represent the same stage of primary seg- 

 mentation in crustacean ontogeny, though metamerism has not yet 

 affected the ectoderm. It is reasonable then to infer, as contended by 

 Iwanoff (1928), that the first somites in both the annelids and the 

 arthropods were formed directly in the previously unsegmented body 

 of the animal. The later extension of the body took place by the 

 teloblastic generation of secondary somites from a subterminal zone 

 of growth. The annelid and arthropod ancestors did not diverge until 

 this method of anamorphic growth was fully established. 



While the three larval forms discussed above do have a basic simi- 

 larity of structure, which is primitive, it is evident that distinctive 

 characters of more recent phylogenetic evolution have been impressed 

 separately on each. The protaspis shows distinctly the definitive 

 trilobite type of structure, the nauplius is clearly a crustacean, the 

 trochophore is a young worm. The trochophore and the nauplius, 

 moreover, are adapted in quite different ways for swimming at an 

 early ontogenetic stage. The trochophore is not an adult ancestral 

 form of the annelids, nor is the nauplius an ancestral form of the 

 Crustacea, 



The metanauplius. — The nauplius is the direct product of em- 

 bryonic development. The further growth of the larva, or of the 

 embryo if hatching occurs at a later stage, proceeds from a subterminal 

 zone of grozvth, which becomes active before the naupliar cuticle is 

 shed, so that rudiments of the new segments may be seen in the 

 posterior part of the body of the nauplius (fig. 2 A). In the meta- 

 nauplius (B), which appears after the last ecdysis of the nauplius, 

 the posterior part of the body is much lengthened; it is now distinctly 

 segmented and bears the rudiments of several pairs of new append- 

 ages. The postmandibular somites are the teloblastic segments 

 (tbSegs). 



