DEGKADATION OF THE PERIECT PAKASITE. 349 



creatvu-e is transformed iuto a mere parasiiical pcuch-like 

 mollusc living in a Holothurian. The Cirrhipedia, Copepoda, 

 and Isopoda among the Crustacea ofler numerous examples of 

 equally extensive degeneration. The extraordinary Thomp- 

 sonia glohosa, described by Kossmann (see p. 47), is nothing 

 more than a small perfectly closed sac attached by a short stalk 

 to the leg of a crab, Melia iesselata ; it contains larvae in the 

 Cypris-stage without a trace of any other organ whatever, and 

 the other two above-named orders of Crustacea contain several 

 equally degenerate forms. In the same way the larvaj of many 

 parasitic worms are often more highly organised than the 

 fully grown and sexually mature individuals, and in many 

 other groups of animals between those here mentioned ' degene- 

 rate metamorphosis ' often appears simultaneously with a para- 

 sitical mode of life. 



Now, at the first glance, it seems tolerably easy to explain 

 the gradual disappearance of many organs in these different 

 creatures by the principle of disuse. We know that a muscle 

 which is not constantly exercised in the proper way gradually 

 loses its power and precision and at the same time materially 

 diminishes in size. The organs of sensation may be rendered 

 keener in their perception by use, and our mental activity in- 

 creases with exercise and diminishes by lack of employment. 

 Thus, applying this principle to the foregoing cases, we might say 

 that the Entoconcha or parasitic crustaceans had lost their 

 organs of motion because, after attaching themselvas to their host, 

 they no longer used them. In the same way we might under- 

 stand the disappearance of a true stomach in Sacculina and 

 Thompsonia, since it becomes useless from the moment when 

 the animal establishes itself in the cavity of its host and by 

 plunging a sucker into its body is enabled to suck up the absorb- 

 able juices of its host, and so to convey them into its own body 

 cavity, without any circuit vid a stomach. Eyes and ears, 

 brain and nerves, muscles and other similar organs, dependent 

 on the will of the animal, might in the same way easily have 

 become extinct from desuetude. But, plausible as all this 

 sounds, certain not unimportant difficulties seem nevertheless to 

 stand in the way of this method of explanation. . An investiga- 



