igog.] N. xA.NNANDALE : The Indian Cirripedia Peditnculata. 65 



In the lyepadidae, however, these plates (or valves, as they are called) have no sooner 

 attained in the course of their evolution a definite number, form and relationship to 

 one another, than their history takes another course and they commence to break up 

 and disappear. I have already remarked that even within the limits of this family it 

 is possible to distinguish different groups in which this degeneracy of the valves does 

 not appear to be due to a common ancestry but rather to the influence of a similar 

 environment on forms which stand to one another in the relationship of distant cousins 

 rather than ancestors and descendants. The degeneracy is not, however, of exactly 

 the same nature in all cases. 



In one species of the genus Lepas (L. tenuivalvata) the valves have almost 

 disappeared owing to their lack of calcification. Their shape, however, still remains 

 the typical one ; they show no tendency to split up, and they cover practically the 

 whole of the capitulum. 



In the genus Dichelaspis a nicely graduated scale of progressive degeneracy occurs. 

 In the most primitive species {e.g., D. tridens and D. hathynomi) the valves are still 

 calcified and cover the greater part of the capitulum. The scutum, however, 

 is split (as it is in several species of Pœcilasma) into two distinct segments, which 

 diverge from one another slightly above, while the part of the same valve that 

 is directed towards the carina has commenced to disappear to a degree differing 

 in different individuals of the same species. Other species of the genus {e.g., 

 D. warwickii) still have a considerable part of their capitulum covered by the 

 valves, but the two branches of the scutum are more divergent and the tergum 

 is much reduced in size. In such species also the extent of the degeneracy is a 

 variable character. Again, we find other species {e.g., the typical form of D. grayii) 

 in which the area covered by the five valves, which are still present, is relatively 

 much smaller, and finally we arrive at forms in which some (or, in one case, all) of 

 the valves have disappeared as calcified plates. In some specimens, however, of even 

 the most degenerate forms it is generally possible to trace the outlines of the original 

 five valves as raised lines on the integument, and often even the concentric lines on 

 the calcified plate, representing the lines of growth, can still be traced on the soft 

 tissues of the capitulum. This is the case as regards some individuals of D. grayii 

 var. pernuda, in which there is no trace of calcification on any part of the capitulum. 



Degeneration of the valves has taken a third course in Conchoderma, in which the 

 membrane has become stout and the valves are very small (although never split up 

 into secondary valves), altogether absent or devoid of calcium salts, while in H et er a- 

 lepas, in which the thickening of the membrane is accompanied by a strong develop- 

 ment of muscular tissue, the valves have practically disappeared, only the scutum 

 persisting as a chitinous, iM-defined plate, and this only in some species. In neither of 

 these genera is it ever possible to distinguish the outline of the original five valves 

 on the capitulum. 



These different types of external degeneracy are correlated with distinct differ- 

 ences in the structure of the appendages and therefore form a sound basis for the 

 larger divisions of the family ; it is worthy of note that they are not accompanied 



