332 SYNOPTIC COLLECTION. 



If a barnacle belongs to the pedunculated Cirripedia 

 like Lepas (No. 815), the stalk or peduncle grows long 

 and the shell forms at its farther end ; if, on the other 

 hand, it is one of the sessile Cirripedia, like Balanus 

 (Nos. 816-818), the shell is fastened directly to the rock 

 and the barnacle looks as figured in PL 814, fig. 4. A 

 slightly older stage is represented in fig. 5, with the tho- 

 racic appendages extended. 



The adult Lepas anatifera Linn. (No. 815) is provided 

 with a long contractile stalk by which it is usually attached 

 to some floating object. At the end of this stalk is the 

 shell which is made of five plates. The shell protects 

 the viscera, the mouth with its mandibles and two pairs 

 of maxillae, and the six pairs of feathery thoracic legs 

 which have become transformed into organs for catching 

 food. It was the discovery of these jointed appendages 

 which caused the barnacle to be removed from the Mol- 

 lusca and placed with the Crustacea. 



In the specimens (No. 815), as in the undisturbed 

 living Lepas, these organs are seen extending from the 

 shell ; when the animal is disturbed they are withdrawn 

 and a transverse muscle draws the valves tightly to- 

 gether. 



The segments of the body are very indistinct, while, 

 during the period the animal was becoming adapted to a 

 sedentary life, the abdomen became a mere vestige and 

 its appendages disappeared. 



Lepas, like the Cirripedia generally, has no heart or 

 specialized circulatory organs and probably no respiratory 

 system. 



The more common form of barnacle is sessile, the stalk 

 not being developed. In the specimen (No. 816) several 

 of these barnacles have made their home upon one valve 

 of Pecten. The opening of the pyramidal shell of the 

 barnacle is closed by four valves ; in some of the speci- 

 mens, however, these valves are open and the cirri are 

 extended. These delicate organs are still more plainly 



