VIII 



AKTHKOPODA 



185 



from other Arthropoda by the non-segmentation of the mesoderm 

 (Balfour, 1880). The anus is still situated on the dorsal aspect of the 

 thoracico-abdominal rudiment. It is, however, shifted somewhat 

 forwards as compared with its former position, and will eventually 

 pass into the terminal notch and so on to the ventral surface of the 

 abdominal rudiment; but this does not happen until a later stage 

 has been reached. 



Finally, on the surface of the egg, outside the thoracico-abdominal 

 rudiment, there is to be seen a semicircular ridge very faintly 

 marked. This is the first trace of the head-shield or carapace. 

 When this stage of development is reached the ectoderm secretes a 

 thin cuticle which is detached from 

 the surface of the egg before further 

 growth occurs, and we may interpret 

 this as the first moult or ecdysis, and 

 as marking the 1 completion of a stage 

 of development. 



Now when we survey what is 

 known of the life -histories of other 

 Crustacea we find that, in the majority 

 of Copepoda, Cirripedia, and Ostracoda, 

 and in the more primitive Phyllopoda 

 as well as in a few Schizopoda and 

 Decapoda, the embryo, when it has 

 attained this stage of development, 

 bursts the egg-shell and escapes as a 

 free -swimming larva, to which the 

 name Nauplius has been given, and 

 which is distinguished by possessing a 

 large upper lip and only three pairs of 

 "appendages. We can scarcely doubt 

 that the formation and exuviation 

 of this cuticle in the embryo of 

 Astacus is a reminiscence of a con- 

 dition when the embryo, at this stage of development, became, 

 a free-swimming Nauplius larva and we regard this as one of the 

 many proofs that the embryonic phase of development is secondarily 

 derived from the larval, and not vice versa. We therefore term this 

 stage of development the Nauplius stage. 



Following the Nauplius stage a different form of development 

 begins ; the ventral shield, which had been undergoing- contraction, 

 begins to grow vigorously, and in the angle where this shield passes 

 into the ventral surface of the thoracico-abdominal rudiment there 

 is found a zone of rapidly growing cells, and as a result of their 

 activity the point of origin of this rudiment is carried farther back. 

 Thus it becomes bent under the ventral shield, just as a crab carries 

 its abdomen permanently bent underneath it. 



Then if we turn our attention to the appendages, we find that the 



FIG. 133. Three transverse sections 

 through the developing nerve cord of 

 Astacusfluviatttis. (After Reichen- 

 bach.) 



comm, mass of cells derived from ventral 

 groove destined to form the transverse 

 commissure ; y, thickening to form one of 

 the ventral ganglia ; v.g, ventral groove. 



