932 Marvels of the Universe 
The scales themselves are marvellous, both in structure and in the charming colour effects which 
they produce ; and, as we have now seen, they cover still other hidden marvels within the wing- 
structure. 
The wings themselves, when denuded of their scales, appear as nothing more than frameworks 
of hollow tubes and ribs, covered with an upper and lower transparent membrane; but when 
we see these apparently simple pieces of mechanism in use, as the butterfly soars over the flowers 
in the sunlight, or flashes its colours along the woodland glades, we have to recognize that even 
they, too, are structures none the less marvellous, although their marvels appear in the finest adjust- 
ments of combined strength, lightness and utility. 
Finally, everyone who has seen a butterfly or moth emerge from its chrysalis and quickly unfold 
and fill out its wings, with all their myriads of coloured scales, every one of which is placed in perfect 
order, will readily agree that 
there are still some extraordin- 
ary details of these insects 
which need explanation. 
PEA CRAB AND 
MUSSEL 
BY EDWARD STEP, F.L.S. 
AmonGa the most  anciently 
known of the strange partner- 
ships that exist between crea- 
tures unrelated to each other, 
there is none more remarkable 
than that subsisting generation 
after generation between the 
Photo by] LW. B. Randtes, little Pea Crab and v amntOUS 
THE PEA CRAB species of bivalves. The ancient 
Shown as it is usually to be found, that is in the shell of the mussel. Both philosophers dwelt upon the 
mussel and Crab benefit by this partnership, probably because the Crab feeds on the o ! : b 
larger organisms which enter the shell of the mollusc and, defying the mussel’s fact, anc gave their own. inter- 
digestion, would yet decay in the shell and set up a diseased condition. pretation of it: and the poets 
of antiquity enshrined it in immortal verse. Naturally, they differed in their explanations, and 
some of them disagreed about the facts; but they did agree that this strangely assorted couple 
lived together constantly in an association that was only severed by the death of one of the two. 
In recent times the story has gained additional interest, because it has been found to be only one 
of a large number of partnerships of similar character, to which modern science has given the name 
of Commensalism—which signifies table companionship ; in another word, messmates. 
The Pea Crab is so-called on account of his size, for he is, when mature, one of the smallest of 
the crabs. He is not one of those that may be found upon the seashore by turning over stones or 
hunting among the seaweeds. If you wish to make acquaintance with the Pea Crab as a living 
entity you must dredge for large-sized mussels, especially the Horse Mussel and the Fan Mussel. 
It was the latter that was well known to the ancients in this connection, the Fan Mussel being a 
differ 
from crabs that live the ordinary free life of their class, in not having their shelly coats armed or 
common bivalve in the Mediterranean. The Pea Crabs—of which there are several species 
decorated by spines, hooks, or bristles. Instead, all their spines have been suppressed, their sharp 
edges rounded off, and the rough exterior smoothed and polished. There are two reasons for this 
uncrablike condition : first, such roughness of exterior and defensive spines are quite unnecessary to 
a crab that lives wholly under the protection of the mussel’s much harder roof ; and second, they 
