CLASSIFICATION OF CRUSTACEA. 273 



abdomen a long penis projects. At the base of this lies the anus. 

 Around the body there is a fold of skin, and from this arise fiye calcare- 

 ous plates, an unpaired dorsal carina, two scuta right and left anteriorly, 

 two terga at the free posterior end. The nervous system consists of a 

 brain, an cesophageal 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 maxillee, 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 oviducts pass to the 

 first thoracic legs, where they pass 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 larvae escape from the 



FIG. 89. Acorn shell (Balanus tintinnabulum). 

 (After DARWIN.) 



^, Tergum ; s, scutum ; d, opening of oviduct ; _/J mantle cavity ; x, 

 depressor muscle of tergum; g, depressor muscle of scutum; h, ovi- 

 duct ; r, outer shell in section ; a, adductor muscle of scuta. 



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, 

 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 Gerard (1597), 

 regarded the barnacle as somehow connected with the barnacle goose, 

 18 



