CRUSTACEANS 



417 



the bifurcated type which we have found to be characteristic of 

 higher forms. There is no gastric mill, and it is common to find 

 three functional eyes present in the adult, two compound and one 

 simple. As a general rule the 

 embryo hatches out as a Nau- 

 plius larva (fig. 258), which typi- 

 cally possesses an unsegmented 

 ovoid body and three pairs of 

 appendages by means of which 

 it swims, these corresponding 

 to the antennules, antennae, and 

 mandibles of the adult. A larva 

 of this kind is rarely found 

 among higher Crustacea. 



The four included orders 

 have already been enumerated ; 

 i.e. I. Barnacles (Cirripedid); 

 2. Bivalve Crustacea {Ostra- 



codd) ; 3. Fork-footed Crustacea ( Copepodd) ; and 4. Leaf- footed 

 Crustacea {Phyllopoda). 



Fig. 258. — A Nauplius larva (seen from below and greatly 

 enlarged). Note unpaired eye and three pairs of appendages 



Order i. — BARNACLES (Cirripedia) 



This is a remarkable group of marine Crustacea, all of which 

 are either fixed or parasitic, and have undergone considerable, 

 or, it may be, profound modifications resulting from their mode 

 of life. Leaving out of consideration the degenerate parasitic 

 forms, which will be dealt with elsewhere, there remain the widely- 

 distributed group of Barnacles, of which the best known are the 

 Ship- Barnacle {Lepas anatifera), and the Acorn- Barnacles (species 

 of Balanus), which encrust the rocks between tide-marks on the 

 coasts of Britain. 



The characters of the group will best be understood by briefly 

 describing the Ship- Barnacle {Lepas anatiferd), large numbers 

 of which are often found attached to floating objects which have 

 been cast up on our shores, and which in old days were a serious 

 nuisance to sailors, as they attached themselves to the wooden 

 bottoms of ships in such vast numbers as to impede movement. 



The Ship-Barnacle (fig. 259) is attached by means of a long 

 soft stalk covered with corrugated skin, and upon this is borne a 



Vol. I. 



27 



