THE INVERTEBRATE LEGIONS 115 



COPEPODS: Order Copepoda 



This is the third and last of the small, planktonic crustacean groups. Again, 

 movement is accomplished by means of two large antennas. Usually there is 

 a single large eye in the center of the head, which has given one fresh-water 

 form the name of Cyclops, after the Homeric giant. The body is usuallv pear- 

 shaped, and the females are often seen carrving two little sacs of eggs attached 

 to their abdomens. Calaniis is one especiallv abundant, cosmopolitan copepod 

 which is eaten by fishes and baleen whales. Some copepods are parasitic 

 (fish lice). 



BARNACLES: Order Cirripedia 



Barnacles have been described as crustaceans that lie on their backs surrounded 

 by a shell, kicking food into their mouths with their feet. These "feet" are 

 actually long appendages that form a sievelike net for the trapping of plankton 

 as thev are rapidly thrust in and out of the shell in a grasping motion. Barnacles 

 are the only sessile crustaceans and are hermaphroditic (both sexes in one 

 animal). Hermaphroditism is present in many sessile animals, doubling the num- 

 ber of voung produced as the result of one copulation. Sessile animals must 

 produce high numbers of voung at a breeding because they can not move from 

 place to place and are thus restricted in acquiring mates. In the case of barnacles 

 the usefulness of hermaphroditism is particularly well illustrated. Unlike many 

 sessile animals which merely shed their eggs and sperm into the sea, barnacles 

 have internal fertilization in which the very long penis of one barnacle deposits 

 sperm directly in the body of a neighboring barnacle. But the barnacle is 

 limited in its mating by the length of the penis, and isolated barnacles out of 

 reach of other barnacles can not mate, although it is possible that these indi- 

 viduals fertilize themselves. The barnacle thus compensates for being sessile by 

 doubling up, each individual being both fertilizer and fertilized, with twice 

 the number of larvae being produced. 



The goose barnacles, Lepas, attach themselves to substratum by means of a 

 long, muscular stalk. Goose barnacles most often attach to floating objects in 

 the sea, the bottom of ships or to seaweeds. One species attaches to jellyfishes. 

 The acorn barnacle, Balanus, is an extremely common animal of the tidal zones 

 of all oceans. 



Barnacles frequently attach to living animals such as molluscs, crabs, or 

 lobsters, and one barnacle, the whale barnacle, lives exclusively on whales. 

 Barnacles do not harm the animals on which they attach except by slightly 

 hindering movement, but some specialized barnacles have become parasitic. 

 The most remarkable of these is SacctiUna (^g. 37^, a parasite on crabs. The 

 Sacczdina lar\'a bores through the shell of a crab and pours its cells into the 

 blood of the crab. These cells conglomerate on the underside of the intestine, 

 and then the whole cell mass grows into a rootlike system which permeates the 

 crab's whole body. The part of the Sacculina containing the reproductive organs 

 protrudes from the underside of the crab's abdomen. SacciiUna lives on the body 

 of the crab, but it does not kill the crab, only causing parasitic castration. 

 Eventually, the Sacculina drops off, and the crab regenerates its sex organs. 



