IV 



than bags of spermatozoa, but they possess to varying degrees 

 the ordinary organs of the adult in a reduced condition. The 

 male of $.j>c/'on.ii (Fig. 55, A) retains the shape and skeletal plates 

 of the ordinary form, and differs chiefly in its reduced size ; but 

 the more common condition is exhibited by the male of S. 

 rulgare (Fig. 52, A), where the scutes are reduced to vestiges 

 round the mantle-opening, and almost the whole of the body is 

 occupied by the greatly developed generative organs. (3) In 

 a few species, e.g. S. velutinum and S. ornatum, the individuals 

 are purely dioecious, being either females of the ordinary 

 structure resembling the hermaphrodites of the other Lepadidae, 

 or dwarfed males resembling closely the complemented males 

 described above for >S'. rulf/cur. 



The nature and derivation of these various conditions will be 

 discussed when the parallel cases found in 

 Ibla and among the Rhizocephala have 

 been described. 



The remaining genus of the Polyas- 

 pidae, also characterised by the presence of 

 numerous skeletal plates on the capitulum, 

 is Lithotrya (Fig. 56), which bores into 

 rocks and shells, and is an inhabitant of 

 the warm and tropical seas. 



The peduncle of the full-grown animal 

 is completely imbedded in the rock or shell 

 to which it is attached, and at the basal 

 end of the peduncle is situated a cup com- 

 posed of large irregular calcined pieces. 

 This cup is, however, not formed until the 

 animal has ceased to burrow. The excava- 

 tion of the substratum is effected by means 

 of a number of small rasping plates which 

 cover the peduncle, the whole being set in 

 motion by the peduncular muscles. 



Fam. 2. Pentaspidae. In this family 

 are placed a number of genera, and among them the common 

 Lepas, the species of which possess typically five skeletal plates, 

 viz., a carina and a pair of scuta and of terga, the peduncle being 

 naked. These forms are a later development of Cirripede evolu- 

 tion, and did not come into existence till Tertiary times. Some 



Fi. 



B 



. 56. Lithotrya dorsal is, 

 x 1 . B, Basal calcareous 

 cup : C, cariua ; R, ras- 

 truni ; S, scutum ; T, 

 tergum. (After Darwiu.) 



