288 



PHYLUM ARTHROPOD A. 



the branched lestes, whose vasa deferentia unite in an ejaculatory duct 

 in the penis. From the much-branched ovaries in the stalk, the ovi- 

 ducts pass to the first thoracic legs, where they open into a cement- 

 making sac, opening to the exterior. The eggs are found in flat cakes 

 between the external fold of skin and the body. 



The life history is most interesting. Nauplius larvre escape from the 

 egg-cases, and, after moulting several times, become like little Cyprid 

 water-fleas. The first pair of appendages become suctorial, and, after 

 a period of free-swimming, the young barnacle settles down on some 

 floating object, mooring itself by means of the antennary suckers, and 

 becoming firmly glued by the secretion of the cement glands. During 

 the settling and the associated metamorphosis, the young barnacle fasts, 



r 







nf^l 



FIG. 137. Acorn-shell (Balanus tintinnabulum}. 

 After Darwin. 



^., Tergum ; s., scutum ; d., opening of oviduct, the aperture is not 

 distinct ; _/., mantle cavity ; jr., depressor muscle of tergum ; -., 

 depressor muscle of scutum; //., oviduct; r., outer shell in 

 section; a., adductor muscle of scuta; cr., thoracic legs; 

 i, first plate of outer shell ; /., position of viscera. 



living on a store of fat previously accumulated. Many important 

 changes occur, the valved shell is developed, and the adult form is 

 gradually assumed. While the early naturalists, such as Gerrard (1597), 

 regarded the barnacle as somehow connected with the barnacle-goose, 

 and zoologists, before J. Vaughan Thompson's researches (1829), were 

 satisfied with calling Cirripedes divergent Molluscs, we now know 

 clearly that they are somewhat degenerate Crustaceans. We do not 

 know, however, by what constitutional vice, by what fatigue after the 

 exertions of adolescence, they are forced 1o settle down to sedentary 

 life. 



The food consists of small animals, which are swept to the mouth by 

 the waving of the curled legs. Growth is somewhat rapid, but the 

 usual skin -casting is much restricted, except in one genus. Neither the 



