304 PHYLUM ARTHROPODA. 
anteriorly, two ¢evga 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 (coelomic) cavities to the base of the second maxillze, 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. 
Fic. 158.—Acorn-shell (Bdlanus tintinnabulum). 
—After Darwin. 
T., tergum ; CR., thoracic legs; F., outer shell in section ; D., aper- 
ture of oviduct; #., mantle cavity; ¥., depressor muscle of 
tergum; AX., antenne; OV., ovary; G., depressor of scutum; 
#., oviduct ; AM., adductor muscle of scuta; S., scutum. 
The life history. Nauplius larve 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 
