THE CIRRIPEDIA. 255 



tral compartments are overlapped on both sides, and their 

 wings are consequently both alae ; the lateral and carino- 

 lateral compartments are overlapped on one side, and overlap 

 on the other, hence they have an ala on one side, a radius on 

 the other ; while the rostro-lateral compartment overlaps on 

 both sides, and hence its wings are both radii. In Balanm^ 

 however, the rostrum and rostro-lateral compartments being 

 replaced by a single compartment formed by their confluence, 

 this piece has radii on both sides. 



Different as is the appearance of Lepas from that of 

 Balanus, they closely resemble one anotner in essential 

 structure. Thus, to commence with Lepas. On cutting away 

 the scutum and tergum of one side (Fig. 67, -S), the hinder 

 part of the body of the animal is seen within the sac of the 

 capitulum, formed by the valves of the shell, to which it is 

 attached only on the rostral side and inferiorly by a com- 

 paratively narrow isthmus. Immediately behind this point 

 the body widens, to constitute what Mr. Darwin' has termed 

 the prosomi^ but the thoracic segments, which succeed the 

 prosoma, gradually taper posteriorly. Six pairs of appendages 

 {a) are attached to the thorax, each limb consisting of a basal 

 joint (protopodite), terminated by two long multi-articulate 

 cirri, the representatives of the endopodite and exopodite ; 

 and a rudimentary abdominal segment, terminated by two 

 short caudal appendages, succeeds the thorax, and is pro- 

 duced in a long setose annulated penis (/*). Filamentous 

 appendages depend from some of the thoracic somites, and, 

 projecting from the inner wall of the sac on each side, is a 

 triangular process, the ovigerous frcenum (m). 



The mouth is situated at the posterior part of a protuber- 

 ant mass, seated on the rostral face of the prosoma. This is 

 principally composed of a large, bullate labrum, behind which 

 are a pair of mandibles with large and setose palps, and two 

 pairs of maxillae. Anteriorly, the prosoma passes by a nar- 

 row isthmus into the rostral part of the peduncle, into which 

 it, as it were, expands ; while the posterior margins of the 

 peduncle become continuous with the walls of the sac. 



The^ extremity of the peduncle is fixed by a peculiar 

 cementing substance to the body to which the Lepas ad- 

 heres ; but, if it be carefully detached, thei-e will be found 

 connected with the rostral portion of the surface a pair of 

 very minute, singular-looking, organs, consisting of two proxi- 



» " Monograpli of the Cirripedia," 1851, 1854. 



