284 TEXTBOOK OF ZOOLOGY 



body length is also about one-tenth inch. It has a single median 

 eye, and the females frequently are seen with a pair of egg sacs 

 attached at the base of the abdomen. Diaptomus, another Copepod, 

 is a common form of about the same form and size as Cyclops, except 

 that the antennae are nearly as long as the body. Argulws is a genus 

 of Copepods which is parasitic on fish, and the individuals are 

 called fish lice or carp lice. They are flat creatures and are found 

 running around over the scales of their hosts. Some of the other 

 forms are parasitic on the gills and fins of fish and their bodies be- 

 come greatly modified. 



The ostracods are small, swimming, bivalve forms that are some- 

 times called swimming clams. This group has beautifully marked 

 valves; in fact, these animals are the most beautiful found in the 

 plankton. 



Adult barnacles of order Cirripedia bear so little resemblance to 

 other Crustacea that they are usually overlooked as such by the 

 layman. They are completely encased in a thick shell of several 

 sections and have the general appearance of an oyster or clam. 

 They are sessile in habit as adults, though free-swimming in the 

 larval stage. Their entire life is spent in marine waters. There 

 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. Sacculina (Fig. 404) 

 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 



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

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

 of the phylogenic and embryonic stages of certain Crustacea, may 

 well come at this point. This theory maintains that certain devel- 

 opmental stages or structures of the individual are related to an- 

 cestral conditions. That is, the individual in its development tends 

 to repeat in an abbreviated fashion the history of the development 



