304 



PHYLUM ARTHROPODA. 



anteriorly, two terga at the free posterior end. The nervous system 

 consists of a brain, an oesophageal ring, and a ventral chain of five or 

 more ganglia. There is a fused pair of rudimentary eyes. No special 

 circulatory or respiratory organs are known. Two excretory (?) tubes 

 lead from (ccelomic) cavities to the base of the second maxillae, and are 

 probably comparable with shell glands and with nephridia. There is a 

 complete food canal and a large digestive gland. Beside the latter lie 

 the branched testes, 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. 



AM 



0V 



FIG. 158. Acorn-shell (Balanus tintinnabulum}. 

 After Darwin. 



7\, tergum ; CR., thoracic legs ; /?., outer shell in section ; D., aper- 

 ture of oviduct; F., mantle cavity; X., depressor muscle of 

 tergum; AN., antennae; OV.> ovary; ., depressor of scutum; 

 //., oviduct ; AM., adductor muscle of scuta ; S., scutum. 



The life history. Nauplius larvse escape from the egg-cases, and, 

 after moulting several times, become like little Cyprids. 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, living on a 

 store of fat previously accumulated. Many important changes occur, 

 the valved shell is developed, and the adult form is gradually assumed. 



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 ecdysis is much restricted, except in one genus. Neither the 

 valves, nor the uniting membranes, nor the envelope of the stalk, are 



