LARVAL STAGES OF TRILOBITES 189 



animal. Afterwards it was found to be a larval stage of 

 Cyclops, but because it agreed in structure with the larvae of 

 many other Crustacea the name was retained for that type 

 of larval form and is now in general use. Primarily it is 

 supposed to represent the first free-swimming stage after the 

 escape of the animal from the egg. However, many species 

 are quite fully developed when leaving the egg, and undergo 

 comparatively slight subsequent metamorphoses, and in these 

 and other species there may be developed in the egg an em- 

 bryo having some of the characters of the nauplius. There- 

 fore the term is also applied to all cases where a certain 

 assemblage of nauplian characters occurs in the development 

 of any crustacean. Thus it may be considered as a stage of 

 development not restricted to a definite period of ontogeny. 



The adult Apus possesses so many nauplian features, and 

 in its development passes through such simple metamor- 

 phoses, that it has been aptly considered by Bernard ll as a 

 nauplius grown to maturity. Balfour 1 also states that the 

 chief point of interest in the development of Apus " is the fact 

 of the primitive Nauplius form becoming gradually converted 

 without any special metamorphoses into the adult condi- 

 tion. " * This form, together with the nauplii of other crus- 

 taceans and the study of the larval and adult characters of 

 the trilobites, ought to afford definite knowledge of the char- 

 acters possessed by the ancestral forms of the Crustacea. 



Before further examining the nauplius it may be well to 

 state the characters which, on the grounds of comparative 

 anatomy and phylogeny, are believed to represent the primi- 

 tive adult crustacean. It will be seen that in many respects 

 the trilobite recalls this type, but, as already suggested, is 

 removed some distance from the prototype, although in itself 

 a most primitive crustacean. Lang 25 gives a very comprehen- 

 sive description of the racial form, as follows : " The original 

 Crustacean was an elongated animal, consisting of numerous 



* The adult Apus properly has five pairs of cephalic limbs. A sixth pair of 

 appendages has been correlated as maxillipedes, though from their innervation 

 they seem to be metastomic and homologous with the chilaria of Limulus. 



