OTHER CRUSTACEA 337 



appendages and the paired ganglia show the existence of a meta- 

 meric structure. The eyes are sense-organs rather than appen- 

 dages, and it is also likely that the antennules were originally 

 tentacle-like sense-organs of the head. The antennse, however, 

 are evidently comparable with the paired appendages upon the 

 remaining somites of the body. It is possible that they were orig- 

 inally posterior to the mouth and have been shifted anteriorly 

 during the evolutionary modification of the Crustacea. 



One may thus picture the schematic crustacean (c/. Figs. 164 and 

 166) as an animal segmented throughout its body except in the head 

 region, having the entire external surface covered by a cuticular 

 exoskeleton, and with a pair of jointed biramous appendages 

 attached to the ventral side of each somite. The mouth is near 

 the anterior end, and the anus opens upon the terminal posterior 



Anknna Brain , ^ ,■ ICdnfix 



- 'jptfcgpngUony- Slomoch Hearf '""""^ r"'^. 



Mouth 



Mondihle _ , ^^ _^ ^^^ .«s5st 



AW///© ^ ^§^ Mi 1st 'W' ^^^ ^^fc ''^^ ^^ ^^ (»»/]«/ 



/*>*■ ' f IS",-- > / 



C cecum ApDen'dogei' l/entrol fierye 



ganglia 



Fig. 164. — Hypothetical ancestor of the arthropods. The parts lal)elecl 

 kidneys correspond to the nephridia of the earthworm. 



(From Pearse, "General Zoology," copyright, 1917, by Henry Holt and Co., reprinted 



by permission.) 



segment. Such an arrangement recalls the type of organization 

 made familiar by our study of the Annulata. For all one can see, 

 the crustacean might be made out of the annulate by a thickening 

 of the cuticle to form a skeleton and by specialization of the para- 

 podia to form the biramous appendages. The resemblance is so 

 obvious that zoologists have commonly assumed the ancestors of 

 the Crustacea, and so of the Arthropoda, to have been closely 

 related to those of the Annulata. 



On the other hand, the internal structure offers little resemblance 

 to the annulate plan of body. There is no clearly recognizable 

 coelome, with its paired nephridial tubules communicating with 

 the outside in each segment and serving as ducts for the re- 

 productive organs. There is only an extensive system of 

 cavities (c/. p. 331) which function as part of the circulatory sys- 



