SIMPLIFICATION WITH SPECIALISATION 293 



to the change which comes over the Ascidian as it grows 

 from the active tadpole stage and becomes converted into 

 this inert, one-purposed sac, but owing to other uses of 

 the word " degeneration " it is better to speak of the 

 condition of the Ascidian as " extreme specialism with 

 simplification." Other very obvious cases of this simplifica- 

 tion connected with specialism are familiar to naturalists 

 among parasitic animals. Most of such animals get abun- 

 dant food by taking advantage of the nutritive juices of 

 the other animals (or the plants) on which they are para- 

 sitic, and whilst developing special apparatus for this 

 purpose, often become reduced and simplified in all other 

 parts of their structure. There are, for instance, a great 

 many kinds of shrimp-like animals which show this 

 condition. When young they are active little shrimps 

 with many legs, claws, jointed body, and muscular tail. 

 But before they have grown to any size the females affix 

 themselves to a fish or other marine animal, and, burying 

 the head and mouth in its juicy flesh, suck in nutriment. 

 They lose their legs and the ring-like jointing of the body 

 and swell out into a sac-like shape. The sac is chiefly 

 occupied by the enormously abundant eggs. When you 

 examine one of these parasites in the adult condition it 

 would not occur to you that a few weeks earlier it was a 

 neat, well-balanced, many-legged little shrimp, darting 

 through the water. Yet such is shown to be its history 

 when we hatch its eggs and watch the early stages of its 

 life. Such is the history, too, of the ship's barnacle 

 (Fig. 40) which begins life as a free-swimming, shrimp- like 

 creature, and then permanently fixes itself by its head 

 (as does the sea-squirt) but retains its legs for the 

 purpose of kicking food into its mouth (Fig. 41). The 

 barnacle is not parasitic, but merely obtains shelter and 

 free carriage by fixing itself to floating wood. 



This tendency to simplification when a special means of 



