§ 269. THE CRUSTACEA. 315 



the number of appendages much less ; so that the three and only pairs, 

 corresponding to the thoracic, are in front of the mouth. With the Cali- 

 gina, and the Ergasilina, the thoracic appendages are prehensile organs, 

 while those of the abdomen are changed into rudimentary oars. With the 

 genus Argulus alone, the first abdominal pair has the form of .suckers,"* 

 the remaining ones beina; fin-like as usual. With the Lernaeodea, the ab- 

 dominal appendages are entirely ^yanting, and there are only a few anterior 

 prehensile ones, two of which, in some genera, are prolonged arm-like, and 

 united, at their extremity, into a button-like, suctorial organ. '■'* Sometimea 

 these arms are wanting, there being present only the suctorial organ.'''* 

 With the Penellina, the locomotive organs are reduced to non-articulated 

 rudiments; or even these may be wanting, and then the cephalic extrem- 

 ity of the unsegmented body has stiff, forked, horny, processes, by means 

 of which these parasites enter the parenchyma of other animals.^'* 



§ 269. 



Certain Crustacea have, moreover, a special locomotive apparatus. 

 With Cypridina, the body is shielded with a bivalve shell, the halves 

 of which move on a kind of hinge. Upon their internal surface are inserted 

 muscular fibres, arising from the back of the animal, which act like the 

 adductor muscles of the bivalve Acephala. 



With the Cirripedia, there is a considerable transverse adductor muscle, 

 which, with the Balanodea, and Lepadea, is situated in the anterior or 

 cephalic angle of the fissure of the mantle, which is nearly always closed 

 by an operculum.'^' In this same angle, the body, with. all the Cirripe- 

 dia, is in connection with the mantle, partly by its cutaneous envelope, 

 which, at this point, is folded in so as to line the cavity of the mantle, and 

 partly by various muscles. These muscles arise from the anterior extrem- 

 ity of the body, which is inverted within the cavity of the mantle, and from 

 both the ventral (or upper), and from the dorsal (or lower) surface of the 

 animal. 



When those of the upper or abdominal surface are contracted, and, at 

 the same time, the adductor muscle of the valves is relaxed, the animal 

 comes out through the fissure of the mantle ; but it is withdrawn into 

 the mantle-cavity when those of the lower or dorsal surface are con- 

 tracted. *-> 



5 Tracheliaxtes, Achtheres, Brachiella. 2 Poli, loc. cit. Tab. IV. fig. 13, y. z. 17 ; Cu- 



6 AnchoreUa. vier, loc. cit. p. 5, fig. 18 b. b., ami Martin St. 



7 Lernaca, Lernaeocera. An^e, loc. cit. p. 1-1, PI. II. fig. 17, 19, J. 

 1 Poli, loc. cit. Tab. IV. fig. 3 J. ; Cuvier, Mem. 



&c. p. 5, fig. 2, 7 e. 11 A, and Martin St. Ange, 

 Mim. &c. p. 15, Pi. U. fig. 18, S. 



* [ § 268.) For a very complete description of Dana, Amer. Jour. Sc. XXXI. 1837, p. 297, and 

 these sucker-like organs, with excellent figures, see Rep. on Crustac. loc. cit. p. 13, 18. — E». 



