254. MARINE INVERTEBRATES 
runs from anterior base to summit, a little back of the margin ; terminal 
end broadly obtuse (truncated) ; dorsal valve much compressed, forming 
a sharp edge serrated with ten to twelve teeth and distinctly striated 
or furrowed. 
Lepas anatifera. A, the entire animal; B, anatomy. a, antennule; ¢, carina; cd, 
cement-gland ; 1, digestive gland ; m, adductor muscle; od, oviduct ; ov, ovary ; p, ped- 
uncle; s, scutum; ¢, tergum and testis; vd, vas deferens; h, tentacles. 
Genus Balanus 
Balanus is known as the “acorn-shell,” or “sea-acorn,” and is 
found in vast numbers and of all sizes between tide-marks, 
incrusting rocks and the piles of piers. It also may be found 
attached to floating objects or to shells, or even to living animals, 
but its usual habitat is stationary. Unlike Zepas, it has no stalk, 
the shell being directly attached to some object. The body is 
surrounded by a fold of skin, to which are attached a shell con- 
sisting of six or more plates and a fourfold lid, or operculum, 
consisting of two scuta and two terga. The operculum may be 
called the door, as the animal opens and shuts it at will and 
has complete protection when it is closed. If one taps a rock 
