368 Mr. G. Chilton on Polymorphism 



long, chiefly echinating the fibre in great abundance ; 3, 

 angulated equianchorate, about 3-6000ths long, confined to the 

 sarcode. Structure throughout uniformly compact, tough, and 

 firm, without axial condensation. Size of specimen 11 in. 

 high by 2\ in. in diameter in its widest part. 



Depth 6 fath. 



Ohs. This is evidently a PlumohaUchondria which, in 

 addition to its self-made or proper spicules, has taken in 

 foreign substances for the axial support of its fibre. 



P.S. — The general forms of the specimens respectively 

 above given can only be taken ccet. paribus as characteristic 

 of the species, since the growth of sponges is so frequently 

 more or less influenced in their form by the environment, 

 that unless a great number of specimens of the same species 

 have been seen it is impossible to determine this accurately. 



[To be continued.] 



XXXIII. — On an Example of Polymorphism in the Amphi- 

 poda. By Charles Chilton, M.A. (New Zealand). 



[Plate X.] 



Several instances of dimorphism in the Crustacea are already 

 known. Fritz Miiller, who seems to have been the first to 

 call attention to examples of it in this group, has given two, 

 one in the Isopoda (Tanais)* and one in the Amphipoda 

 (Orchestia) "l ; in each of these cases there are two forms of 

 the male to one of the female. Mr. G. M. Thomson has re- 

 corded a similar example from the terrestrial Orchestia of 

 New Zealand J. From the examination of a large number 

 of specimens of Orchestia gathered from several different 

 localities in New Zealand he comes to the conclusion that 

 they all belong to one variable species, Orchestia sylvicola^ 

 the " males of which have at least two forms of the gnatho- 

 poda." Another example is found in Moera subcarinata^ 

 Haswell {M. Petriei, G. M. Thomson) ; but here the two 

 forms of the males only ditfer very slightly from one another. 

 In specimens taken by Mr. Thomson at Stewart Island, New 

 Zealand, the males had " the whole lower surface [of the 

 propodos of the posterior gnathopoda] very densely fringed 



* ' Facts for Darwin,' p. 20. t L. c. p. 24. 



\ ' Transactions New-Zealand Institute,' vol. xiii. p. 212. 



