292 



LECTURE Xlll. 



Of this most singular form of Articulate animal, the same acute 

 Observer from whom the above passages are quoted has determined 

 analytically the common pattern on which the cirripedial modifications 

 have been superinduced. The archetype consists of seventeen seg- 

 ments, the first fourteen of which, as in the Crustacea, constitute the 

 cephalo-thorax ; the other three are abdominal segments. The two 

 pairs of antennae and the eyes determine the first three segments of 

 the head ; the three pairs of jaws mark the next three segments ; and 

 the seventh cephalic differs from the succeeding seven thoracic seg- 

 ments. In all the common cirripeds two segments disappear : these 

 Mr. Darwin believes to be the last cephalic and the first thoracic ; 

 and the six pairs of cirri belong therefore to the six last thoracic 

 segments. 



The general analogy of the pedunculated cirriped to the typical 

 Crustacea, and the true nature of the peduncle, will be best understood 

 by the diagram reduced mjig. 128, from Mr. Darwin's monograph. 



A. Leucifer. 



B. Lepas. 



The upper figure is of a stomapod crustacean : the abdomen, which 

 in cirripeds is rudimentary, is given in outline. The lower figure is 

 a mature Lepas, with the outer antennae {a) and eyes (e), which are 

 actually present in the larva, retained and figured of proportionate 

 size. All that we see externally of a cirriped, whether pedunculated 

 or sessile, is the three anterior segments of the head of a crustacean, 

 with its anterior end permanently cemented to a surface of attachment, 

 and with its posterior end projecting vertically from it. 



The capitulum, or shell, answers to the carapace in higher crusta- 

 ceans, which is a backward production of the tergal elements of the 

 third segment. It is divided into {s) the scutum, (t) the tergum, and 

 the dorsal line of splitting is defended by the narrow carinal piece c ; 

 b is the basis of the capitulum, compound carapace or shell ; m, 

 the position of the mouth ; p, the termination of the intromittent 



