THE BIOGENETIC LAW 169 



very remarkable attaching organs are sometimes developed, in the 

 form of hooks or of knobbed pincers, or of actual suckers. In several 

 types the degeneration and modification go so far that the segmen- 

 tation of the body disappears, and the animal looks more like an 

 intestinal worm than like a Crustacean {Lermeocera and others). In 

 all these forms adapted to a parasitic mode of life it is always only 

 the mature animal which has been transformed in this manner, for 

 previously it has gone through a series of stages which are quite 

 similar to those of the free-swimming Copepods, beginning with the 

 nauplius, and ending with the so-called Cyclops stage, that is, a larval 

 form which possesses antennae, eyes, and swimming-legs similar to our 

 freshwater Copepods of the genus Cycloids. 



Here again we see in the ontogeny the repetition of a series of 

 phyletic stages before the mature form is assumed. Why these 

 stages should have persisted it is easy enough to understand, for how 

 could an animal which emerged from the egg as a worm-shaped 

 lermeocera find a fresh fish which would serve it as host ? Yet these 

 parasites could not possibly go on preying upon the same fish genera- 

 tion after generation. To secure the existence of the species it was 

 therefore indispensable that the faculty of swimming should be 

 retained at least in the young stages ; in other words, that the free- 

 swimming ancestral stages should be preserved in the ontogeny. In 

 all these cases it is therefore beyond doubt that the germinal history 

 recapitulates a series of stages comparable to those of the racial 

 history, although not quite unchanged but adapted to the modern 

 conditions of life, for instance in having shorter antenna, smaller 

 eyes, and with four instead of the usual five swimming-legs. The 

 search for a host does not seem to last long, for fishes are usually 

 found in large numbers together, and thus the young parasitic 

 Crustacean does not require to make a long journey before it finds 

 a refuge. 



It is noteworthy that the males of parasitic Crustaceans are not 

 only much smaller than the females (Fig. 113), but that they are also 

 much less modified, and resemble the ancestral free-swimming 

 Copepods to a much greater degree. They usually possess small but 

 well- developed swimming-legs, and by means of these they seek out 

 the female, dying after fertilization is accomplished. They are thus 

 not sessile parasites at all, and have therefore to go through the 

 stages of the free-swimming Copepods much more completely than 

 the females, whose task is to accumulate within themselves from the 

 blood of the fish as much material as possible for the forming of the 

 eggs, and to produce the largest possible number of these. These 



