12 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



the fact that the young still resemble the latter in the 

 possession of eyes and antennae, segmented bodies, well 

 developed jaws, and numerous legs : in short, in all essential 

 points of structure, they exactly resemble the locomotive forms. 

 The young of the Entoniscidae are actually free-swimming 

 organisms, and it is necessary for the perpetuation of the 

 species that they should be so, for how could the parent 

 animal, possessing no organs of locomotion, leave its original 

 host for a fresh one ? And yet such a change is essential for the 

 continuance of the species ; for in course of time the hosts will 

 die. Under such circumstances the young Entoniscidae leave 

 the mother as perfect Isopods, make their way out of the host, 

 and lead a free-moving life in the sea until they find and enter 

 another Carcinus maenas : they then undergo a whole series 

 of retrograde changes in rapid succession, and finally attain the 

 remarkable vermiform shape already spoken of Of course, 

 retrogressive development did not reach anything like this 

 degree at first ; it was only attained after the lapse of countless 

 generations, and a passage through many intermediate forms. 

 The original parasitic Isopods lived no doubt, like the fish-lice, 

 attached to the external integument of their host ; these were 

 followed by others which took up their abode in the internal 

 cavities of the body, in the respiratory chamber and the 

 alimentary canal. Gradually increasing modification then 

 occurred, as the parasites found their way farther and farther 

 into the internal organs. The Entoniscidae are not the most 

 extreme cases of retrogressive development among the para- 

 sitic Crustacea ; there are species in which not only the legs, 

 antennae, eyes, and segments of the body, but the whole head, 

 and even the stomach, intestines, and mouth disappear ; food 

 being taken in through peculiar root-like tubes, which absorb 

 the juices of the host in such a manner as to supply ready made 

 nourishment which needs no digestion. But the Entoniscidae 

 afford sufficient proof of the extraordinary effect of the disuse 

 of certain parts in transforming the whole organic structure of 

 a species. 



Since we find that disuse of an organ is always followed by 

 its gradual disappearance in the course of many generations, 

 the supposition naturally arises that this decline is the direct 

 consequence of disuse, and that the inactivity of an organ is the 



