346 



PHYLUM ARTHROPODA 



balanoides) ; others are hermaphrodite, with complementary males 

 (5c. villosum) ; and others are unisexual, but the males are minute 

 and parasitic (5c. regium). 



Balanus, the acorn-shell, encrusts the rocks in great numbers between 

 high and low water marks. It may be described, in Huxley's graphic 

 words, as a crustacean fixed by its head, and kicking the food into its 

 mouth with its legs. The body is surrounded, as in Lepas, by a fold 

 of skin, which forms a rampart of six or more calcareous plates, and a 

 fourfold lid, consisting of two scuta and two terga. When covered by 

 the tide, the animal protrudes and retracts between the valves of the 

 shell six pairs of curl-like thoracic legs. The structure of the acorn- 

 shell is in the main like that of the barnacle, but there is no stalk. 



ov Mn 



Fig. 



190. — Acorn-shell {Balanus tintinnabulum). 

 — After Darwin. 



r., Tergum ; CR., thoracic legs ; R., outer shell in section ; D., aper- 

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

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

 /f., oviduct ; ^.M., adductor muscle of scuta ; S., scutum. 



The life-histor}' also is similar. A Nauplius is hatched. It has the 

 usual three pairs of legs, an unpaired eye, and a delicate dorsal shield. 

 It moults several times, grows larger, and acquires a firmer shield, a 

 longer spined tail, and stronger legs. Then it passes into a Cypris 

 stage, with two side eyes, six pairs of swimming legs, a bivalve shell, 

 and other organs. As it exerts itself much but does not feed, it is not 

 unnatural that it should sink down as if in fatigue. It fixes itself by 

 its head and antennae, and is glued by the secretion of the cement 

 gland. Some of the structures, e.g. the bivalve shell, are lost ; new 

 structures appear, e.g. the characteristic Cirriped legs and the shell. 

 Throughout this period, which Darwin called the " pupa stage," there is 

 external quiescence, and the young creature continues to fast. The skin 



