APPENDIX. 39 1 



fecfcly simple, straight intestinal canal is visible, possessing a 

 mouth in front, and an anal orifice behind. In front, above the 

 mouth, lies a simple, single eye. All the six foi'ms of nauplius 

 entirely agree in all these essential characteristics of organiza- 

 tion, whereas the six fully developed forms of Crustacea belong- 

 ing to them, Plate XI., are extremely different in organisation. 

 The differences of the six nauplius forms are confined to quite 

 subordinate and unessential relations in regard to size of body, 

 and the formation of the covering of the skin. If they could 

 be met with in this form in a sexually mature condition, no 

 zoologist would hesitate- to regard them as six different species 

 of one genus. (Compare vol. ii. p. 175.) 



Plate XI. represents those fully developed and sexually mature 

 forms of Crustacea, as seen from the right side, which have 

 ontogenetically (hence also phylogenetically) developed out 

 of the six kinds of nauplius. Fig. A c shows a freely swim- 

 ming fresh-water crab (Limnetis brachyurus) from the order of 

 the Leaf-foot Crabs (Phyllopoda), slightly enlarged. Of all the 

 stiU living Crustacea, this order, which belongs to the legion of 

 Gill-foot Crabs (Branchiopoda), stands nearest to the original, 

 common primary form of nauplius. The Limnetis is enclosed in 

 a bivalved shell, like a mussel. Our drawing (which is copied 

 from Grube) represents the body of a female animal lying in the 

 left shell ; the right half of the shell has been removed. In 

 front, behind the eye, we see the two feelers (antennae), and 

 behind them the twelve leaf-shaped feet of the right side of the 

 body, behind on the back (under the shell), the eggs. Above, in 

 front, the animal is fixed to the shell. 



Fig. B c represents a common, freely swimming fresh-water 

 crab (Cyclops quadricornis) from the order of Oar-legged crabs 

 (Eucopepoda), highly magnified. In front, below the eye, we 

 see the two feelers of the right side, the foremost of which is 

 longer than the hinder one. Behind these are the gills, and 

 then the four paddling legs of the right side. Behind these are 

 the two large egg-sacks, which, in this case, are attached to the 

 end of the hinder part of the body. 



