CRAYFISH 



413 



are several characteristic barnacles, rock barnacles on rocks, whale 

 barnacles from ships and pilings, and gooseneck barnacles of the 

 stalked type. After attachment, the legs become modified into feather- 

 like bristles which are used in gathering food. SaccuUna is a genus 

 related to true barnacles which has gone parasitic on crabs and has 

 lost all resemblance to animal form. It settles on the body of a crab, 

 makes its way to the interior and there becomes a branched mass of 

 tissue which penetrates by roots to all parts of the body of the crab. 

 After a time a baglike portion forms and projects externally on the 

 ventral side of the abdomen of the crab. 



Recapitulation Theory or Biog-enetic Law 



A statement of this idea, which was developed by von Baer, 

 Haeckel, and others, and is so well illustrated by the comparison 



Fig. 167. 



Meg^IopiS 



Fig. 168. 



iSchizopod 



Fig. 169. 



(Courtesy of General 



Rinfrlfi^o-^^o^^'^^T^T^^^ ^^^^^ ^^ the barnacle, Balanus. 

 Biological Supply House.) 



theS^staJesTn °Hfpir^!^l.i^^^^^°P.^ ^.V'^^^^ ^^ developing: Crustacea. Crabs include 

 mese stages in their development. (Courtesy of General Biological Supply House.) 



in tv^if; 169.— Schizopod or mysis stage through which the shrimp and lobster pass 

 in their development. (Courtesy of General Biological Supply House ) '"^^^^^^ ^^^^ 



of the phylogenic and embryonic stages of certain Crustacea, may 

 well be mentioned again at this point. This theory maintains that 

 certain developmental stages or structures of the individual are re- 



