1892.] On the early Development of Cirripedia. 



development ; but the mesoblastic layer becomes thickened locally to 

 form the muscles of the appendages in each of the three segments. 



The body cavity* of the nauplius arises later as a mixed blasto- 

 coale and schizoccele, due in part to the separation of ectoderm and 

 endoderm, and in part to an excavation of mesoblastic tissue. It 

 soon forms a cavity continuous from end to end of the body. 



The nervous system of the nauplius (arising as usual as an epi- 

 blastic thickening) shows from the beginning a complex structure, 

 especially in the Balanids, and among these it is most specialised in 

 the Balaninge. It is probably from the first a syn-cerebrum, since it 

 includes, in addition to the representative of the archi-cerebram, the 

 ganglia supplying the antennules. The antennae and mandibles are 

 in close relation with the circum-oesophageal connectives and sub- 

 cesophageal ganglion respectively. 



A comparatively sudden change is experienced by the nauplius in 

 passing from stage I to stage II ; this necessitates a telescoping of 

 the tail, caudal spine, and bristles of the appendages, the gradual 

 evagination of which 'gives rise to the peculiar appearances seen a^ 

 this stafr~ 7 and which have given rise to some misconceptions. 



There is a most remarkable agreement between the nauplii of the 

 various species in the general structure of the carapace, labrum, &c., 

 extending to the minutest detail in the case of the appendages, and 

 indicating that the features in question have been inherited from 

 some stage of the common ancestor. 



There are, however, points of difference which concern chiefly the 

 carapace (with its horns and caudal spine), labrum, and tail. 



Differences are perceptible in most cases in the new-laid ovum of 

 different forms, and the genera, or even the species, can thus be 

 separated even at this early stage. 



The larval differences necessitate a classification which agrees very 

 closely with that deduced from the structure of the adults. 



Such differences have, however, in most cases been acquired inde- 

 pendently of adult structure, since they concern characters peculiar 

 to the larvae, and lost by the adult. Some indications of the preco- 

 cious appearance of characters originally belonging to the later stage 

 are seen in Balanus ; but most of the characters cannot be so ex- 

 plained. The larva and adult have varied simultaneously, but in 

 quite different ways, each having in this group taken its own course 

 of evolution. 



The agreement in the development of such forms as Balanus and 

 LepaSj stage by stage, indicates that the ancestor of the Thoracica 

 underwent a metamorphosis similar to that of the present members 

 of the group. The Nauplius and Cypris stage have, therefore, not 

 been evolved within the group. 



* This term is used in a purely descriptive sense. 

 VOL. LII. M 



