IGO CRUSTACEA. 



individual cases it is often impossible to distinguish between these 

 ectodermal ingrowths and true connective tissue. This inner 

 chitinous supporting tissue belonging to the ectoderm was found 

 by Claus (No. 9) ver}' richly developed in Branchipus. 



B. Endoskeleton. 



A further development of internal chitinous structures is brought 

 about by infolding and invagination of the external integument. 

 In this way are developed those tubular chitinised ingrowths which, 

 as affording attachment for the more important muscles, have been 

 called chitinous tendons, and some of which even, as was proved for 

 the mandiljular muscle of Astacus (Baur), are renewed at ecdysis. 

 A striking development of chitinous tendons of this kind, formed 

 from ectodermal invaginations (Eeichenbach), is found, as is well 

 known, in the penultimate joint of the pincer in Astacus fliiviatilis. 

 The inner sternal skeleton (endophragmal system) which bridges over 

 the thoracic ganglia in Astacus develops in a similar manner, as a 

 series of invaginations of the external integument (Bobretzky, No. 

 41), by a process of infolding of the inner wall of the branchial 

 cavity and of the sternal surface of the thorax. Nusbaum (No, 85) 

 was able to observe in Oniscus the origin from paired lateral invagi- 

 nations of a similar chitinous diaphragm covering over the chain of 

 ganglia in the thorax. A hemispherical, chitinous articular fold also 

 arises as an ectodermal invagination in connection with the movable 

 eye of the Cladocera and of most Branchiopoda (Grobben). 



0. Nervous System. 



Although probably belonging to a common rudiment,* the supra- 

 oesophageal ganglion (brain) and the vential chain of ganglia must 

 be separately dealt with. The whole of the central nervous system 

 arises as an ectodermal thickening. Even in early stages, paired 

 ectodermal thickenings can be recognised on the inner side of the 



* Most authors agree that, in the Crustacea, the rudiment of the brain, from 

 its lirst appearance, is connected with the primitive swellings of the ventral 

 chain of ganglia by means of paired ectodermal thickenings (rudiments of the 

 oesophageal commissures). This view, however, has been opposed. UiiBANOWicz, 

 for example, found that in Cyc/ops (No. 23) the brain and a suboesopliageal 

 ganglion originate independently, and only become connected later liy the 

 development of the oesophageal commissures. Tliis observation cannot, however, 

 be considered as affording direct proof of Kleinenberg's views as to the original 

 independence of the rudiments of these two parts of the central nervous system 

 (Vol. i., p. 288), for it is easy to understand that the rudiments of the more 

 massive parts of tlie central nervous system should become earlier perceptible as 

 ectodermal thickenings, while those of the more delicate parts {e.g., of the 

 oesophageal commissures) are only visible at a later stage. 



