28 



DEGENERATION 



a certain shrimp (Figs. 4, 5, 6). Let us now compare 

 these with the young stages of a number of shrimp- 

 like animals, viz. Sacculina, Lernseocera, Lepas, Cy- 

 clops, Limnetis (all drawn in Fig. 4), some of which 

 lead a parasitic life. All start equally with the re- 

 capitulative phase known as the Nauplius ; but whilst 



Fig. 7. — Adult Sacculiua 

 (female). 



Fig. 8. — Adult female 

 Lerngeocera. 



the Nauplius of the free-living shrimp grows more and 

 more elaborate, observe what happens to the parasites 

 — they degenerate into comparatively simple bodies ; 

 and this is true of their internal structure as well as 

 of their external appearance. The most utterly re- 

 duced of these parasites is the curious Sacculina 

 (Fig. 7) which infests Hermit-crabs, and is a mere 

 sac filled with eggs, and absorbing nourishment from 

 the juices of its host by root-like processes. 



