or cling on duckweed or other submerged vegeta- 

 tion by use of the same slender swimming legs that 

 can be extended through the gap in the shell. C\[)ri.s 

 is one of the common genera. 



BARNACLES 



Barnacles, which are crustaceans too (order 

 Cirripedia) go through first a nauplius stage and 

 then a bivalved form called a "cyprid"" stage because 

 of its resemblance to an ostracod. It is the cyprid 

 stage that attaches itself to some solid object by 

 means of the first pair of head appendages. There- 

 after for a short time the young barnacle ceases to 

 feed. It transforms itself into the degenerate adult 

 form, secreting around its body a hard limy shell 

 composed of several separate movable pieces. Inside 

 this shell the barnacle is a prisoner, unable to move 

 from place to place. It is attached to its covering by 

 the region corresponding to the back of its neck, a 

 fact that led an eminent biologist to describe a bar- 

 nacle as an animal lying on its back, kicking food 

 into its mouth with its feet. Much of what is known 

 about barnacles comes from the monumental re- 

 search work of Charles Darwin — work that estab- 

 lished his reputation as a professional scientist fully 

 a decade before the appearance of The Origin of 

 Species in 1859. 



Wharves and pilings, as well as ship bottoms and 

 floating timbers, often bear a lively covering of 

 gooseneck barnacles (Lepas), each holding to the 

 support by means of a flexible leathery stalk that 

 holds the shell-covered portion of the animal out into 

 the surrounding water. Gooseneck barnacles usually 

 have a shell composed of five limy plates, and pre- 

 sent a somewhat flattened appearance suggesting a 

 strange kind of clam attached by its siphon. 



Barnacles attached to rocks are more often of the 

 acorn type (Balanus), in which a conical ring of four 

 closely fitted plates is closed at the top by two addi- 

 tional movable plates. When the latter are spread 

 apart, the animal reaches out feather-like feet and 

 combs the adjacent water for microscopic food parti- 

 cles. These are pulled into the cavity of the shell and 

 there transferred to mouthparts that consolidate the 

 catch and pass it to the mouth proper. 



Both plankton organisms and the detritus from 

 partial decomposition of living matter are acceptable 

 food to barnacles. Upon a diet of this kind the acorn 

 barnacle Balaims mihilus of Puget Sound on North 

 America's west coast reaches a diameter of nearly a 

 foot, and becomes an attractive item of food for hu- 

 man beings. 



Man usually regards barnacles as "fouling" organ- 

 isms that fasten themselves to ship bottoms and 

 greatly increase the frictional drag of the hull upon 

 the water. Barnacles also find a place on or in the 

 skin of whales. Coronula, the commonest barnacle of 



This fresh-water ostracod, whose head end is at the 

 left, is less than i '-^r, inch across its enclosing bivalved 

 shell. Antennae and legs, which can be extended be- 

 tween the valves, beat and kick the animal along. 

 (Illinois. P. S. Tice) 



whale skin, may reach a diameter of 3 inches. Often 

 this acorn barnacle, in turn, supports a few of the 

 gooseneck barnacle Conchoderma, which is unusual 

 in that it has only minimal shell covering and its feed- 

 ing organs are enclosed in little hoods opening in the 

 direction toward which the whale swims. From the 

 appearance of its paired hoods, Conchoderma is 

 called the rabbit-eared barnacle. 



The step from being an embedded barnacle to one 

 living a parasitic life may not be very big. It has been 

 taken by a number of cirripedes, which thereby be- 

 came "root-headed barnacles." The best known of 

 them is Sacciilina. since it attacks a variety of crabs, 

 including the widespread green crab Carcinides mae- 

 nas of the North Atlantic's shores and several differ- 

 ent common kinds on the Pacific coast of America. 



Sacciilina reaches its victim while still in the free- 

 swimming cyprid stage, but thereafter follows a truly 

 amazing course. The Sacciilina larva pierces a hollow 

 bristle on the crab's body and through it frees into 

 the crab's blood stream a few cells that float about 

 inside the victim until they come to rest at the junc- 

 tion between the crab's stomach and intestine. There 

 the cells attach themselves and grow on nourish- 

 ment from the crab's blood while extending a mass of 

 rootlike processes throughout the body of the host. 



The growing Sacciilina parasite invades and de- 

 stroys the crab's reproductive organs. This not only 

 terminates the host's ability to reproduce but alters 

 its sex hormones until, at the next molt, it assumes 

 female body form regardless of its inherited sex. Fe- 

 male body form includes an apron-shaped abdomen, 

 which effectively protects Sacciilina when the latter 

 creates an opening in the crab's body wall on the 

 under side at the base of the abdomen and extrudes a 



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