SECT. XI DEVELOPMENT 



beyond the Trochophora stage. A comparison of 

 the Nauplius of Apus just hatched from the egg 

 with the figure of a Polychsetan Trochophora shows 

 at a glance that the former stands at a far higher 

 stage of development than the latter. That this is 

 in reality the case Is also clear from the fact that 

 the Nauplius develops three limbs, i.e., the homo- 

 logues of the parapodia of the first three Annelidan 

 segments, traces of the dorsal fold which belongs 

 to the fifth segment, and further slight indications 

 of five trunk segments (see Fig. 34), in all ten 

 Annelidan segments. A larva so far developed 

 cannot be compared with the simple ciliated Tro- 

 chophora, which when hatched probably represents 

 only tivo, i.e. the first and last, segments of the adult 

 Annelid. 



With certain characteristic differences for each 

 group, the Nauplius Is essentially the same through- 

 out the whole class of the Crustacea. Its exact 

 morphology we shall endeavour to explain with the 

 aid of the light we have now obtained as to its 

 origin, as the larva of Apus, or the Apus-stage in 

 the development of other Crustacea. 



When hatched from the <^^^ the Nauplius has 

 three pairs of Crustacean limbs, the unpaired " eye," 

 the dorsal shield, the large upper lip, and what is 

 not usually mentioned — the bent intestine, or, what 

 is the same thing, the rudiments for the development 

 of such a bent intestine (see Fig. 37). Wc will 

 take these points In turn. 



I. The Nauplius Limbs. — We are not bound to 



