418 



CRUSTACEA ENTOMOSTBACA. 



In the Operculata or sessile barnacles the anterior part of the 

 body does not undergo this elongation, but increases very largely 

 in width, its " basis " or disc of attachment being the widest part 

 of the body. In them the region of the body morphologically 

 posterior is sunk in the wide anterior part, being surrounded by 

 a fold of the outer wall (Fig. 272 6). Calcareous plates developed 

 in this fold and iji the wall of the anterior part below it con- 

 stitute the shell a^testa. The calcareous plates of the Operculata 

 are described Iqraow, but it may here be pointed out that the 



f-Tu 



FIG. 272. a, Lepaz after removal of the right shell ; b, Balanm tintinnabulum (after 

 Ch. Darwin), one half of the shell has been removed. The following references apply 

 to Fig. 272 a and b, and Fig. 273. A' anterior antennae at the end of the stalk in a ; 

 Ad adductor muscle ; C carina ; Cd Cement gland and duct ; Cf thoracic appendages ; 

 F caudal fork ; L liver ; M muscle ; Mk oral cone ; Od oviduct ; Oe opening of oviduct; 

 Ov opary ; P penis ; tic scutum ; T testis ; Te tergum ; Tu section of the outer shell ; 

 Vd vas d'eferens. (After Claus.) 



fold hi question is distinct from that forming the mantle, being 

 situated externally to it, and completely surrounding the body. 



After the shedding of the bivalve shell of the pupa the fold 

 of skin which supported it is covered by a thin cuticle in which 

 however five local thickenings of chitin have already appeared. 

 These are the provisional " valves " of the shell ; they are always 

 the first plates to appear, however many maybe present in the 

 adult shell, and in the Lepadidae the calcareous shells of the 

 adult are formed under and about them. 



The adult Lepas, whose relation to the larva may now be 



