THE LIFE CYCLE 



359 



antero posterior polarity, for example, is shown by the in- 

 cised planarian of figure 205 /*, which is producing new heads 

 where the strips of severed tissue are directed forward, and 

 tails where these are directed backward. But the expected 

 does not always happen in regeneration. In at least one 

 genus of earthworms, if any number from one to five of 

 the front segments of the body be cut off, these will be 

 replaced in like number ; but if a dozen or twenty or any 

 number of segments more than five be cut away from the 

 front end, only five will be regenerated in their stead ; and 

 if more than the anterior half of the body be cut away, 

 from the front end of the posterior piece there will develop 

 not a new head but a new tail. Apparently, there is a limit 

 to anterior polarity. 



The hydroid Antennaria regenerates a new head when the 

 decapitated stem is kept in the upright position, but a new 

 foot when it is kept in inverted position. And stems of the 

 hydroid Pennaria, which regenerate heads under ordinary 

 circumstances, will regenerate roots if the cut ends are held 



against a solid support. From 

 the severed eye-stalk of a craw- 

 fish (or almost any other deca- 

 pod crustacean) a new eye 

 never develops, but on the 

 contrary, if there be any re- 

 generation (as there is pretty 

 sure to be if the animal be 

 young, and the conditions 

 favorable for growth), it is 

 usually a jointed appendage, 



FIG. 209. Regeneration of the ^^-^ ^^ l^cc an+^trna 1iV* anrl 



stalked eye of the crawfish (after HlOre Or 16SS antenna-like, ana 



Miss Steele). Ing. a simple appen- -j. Kpof Hpvplnnmfmt rlis- 



dage is regenerated in the place at 1TS Desi Development Q1S- 



of the eye; r, a biramous appen- fi-npfl-rr hi ram one fhaf crrn-wrc 



dage, that regenerated in the tUlCtiy Dl-ramOUS, tnat grOWS 



place of the eye of another speci- Qut ^ the eye > s stead (fig< 2Qg) ^ 



