194 



LEISURE-TIME STUDIES. 



adult structure, exactly resemble each other in their young 

 state, the feasible nature of the statement that such a like- 

 ness implies a common origin, is readily demonstrated. On 

 any other supposition, in short, the development of living 

 beings presents us with phases of utterly unintelligible nature. 

 Now, the young sacculina is found to present a close re- 

 semblance to a large number of other animals belonging to 

 the great class known as the Crustacea. To this group belong 

 the barnacles, water-fleas, fish-lice, shrimps, crabs, lobsters, 

 etc. Most of these animals leave the egg in the form of a 

 " nauplius," and present the closest possible resemblance to 

 young sacculinse. The young of the fixed and rooted bar- 

 nacles (Fig. 5), which attach themselves like pseudo-parasites 

 to the sides of ships, so closely resemble young sacculinae that 

 it would be a difficult, if not absolutely an impossible, task 



to separate or distinguish the 

 young from those of the sac- 

 culina in the earlier stages of 

 growth. The young barnacle 

 (Fig. 22), like the young sac- 

 culina, resembles a shrimp of 

 peculiar kind on a roving com- 

 mission, much more closely 

 than it does the adult and 

 attached form. And hence we 

 discern in the common like- 

 ness of the young of these 

 animals a proof of their com- 

 mon origin. At one time, 

 therefore, we may believe that the sacculina existed as a free- 

 swimming creature, of active habits, and possessing a tolerably 

 high degree of organisation. Doubtless some less energetic 

 member of the sacculina-family secured a temporary resting- 

 place on the body of a crab, and found such a position to be 

 of desirable kind from the rest and protection it afforded. The 

 feelers or feet, which were at first used for mere attachment, 

 may have come in time to penetrate the body of the crab- 



FIG. 22. Young barnacle. 



