io8 



THE CRUSTACEA 



MORPHOLOGY OF THORACICA AND ACROTHORACIGA. 



In general form and in many details of structure the Cirripedia 

 as a whole depart more widely from the common type than do any 

 of the other sub-classes of the Crustacea. This is correlated with 

 the sessile habit of life which is universal within the group. 

 In those forms which have become purely parasitic, as in the 

 orders Ascothoracica, Apoda, and Rhizocephala, the modifications 

 are still more profound, leading, in the last-named order, to the 

 disappearance of every trace of Arthropod organisation. Neverthe- 

 less, the life-history and even the minute characters of the larvae 

 are so constant throughout the group that it is impossible to 

 question the close relationship of the various forms. 



B. 



r+r.l.- 



sc- 



-c. 



c. 



Kio. :>-. 



A, Lrjms anatifera; B, Balunvs licnnrri. (After Darwin.) c, carina ; <:!. o;iriiio-!ut<>ral ; !, 

 lateral ; p, peduncle ; r+r.l, rostrum coalesced with rostro-lati-ial : se, scutum ; /, tergum. 



Leaving aside for the present the parasitic orders named above, 

 the remaining Cirripedia, forming the orders Thoracica and Acro- 

 thoracica (Abdominalia of Darwin), show considerable uniformity 

 of structure, though differing widely in general appearance. In all 

 cases, the animal is attached to some foreign object by the anterior 

 portion of the body in the region of the antennules. The shell- 

 fold is greatly developed, forming a " mantle " enclosing the body 

 and limbs, and strengthened, as a rule, by shelly plates on its outer 

 surface. Owing to a strongly marked dorsal flexure of the preoral 

 region, the greater part of the body within the mantle comes to lie 

 nearly at right angles to the anterior part. The anterior part of 

 the cephalic region may be elongated into a flexible and muscular 

 peduncle (Fig. 57, p), as in the Peclunculata, or represented only by 



