934 Marvels of the Universe 
THE ERYOPS. 
One hint of the reptile is to be found in the presence of blunt spurs on the ribs of Eryops, such as are met with to-day 
only in crocodiles, alligators, the Tuatera, and the birds. The position of the ribs denotes also that this strange creature breathed 
like a frog. 
would be distasteful and positively harmful to the delicate mollusc. So natural selection and the 
survival of the fittest has resulted in the evolution of a smooth and unarmed crab that can jostle 
against the soft tissues of the mussel without doing them any harm. Some may contend that this 
is not so; that it is more reasonable to suppose that the Pea Crab took up residence in the mussel- 
shell because it was unfitted by its unarmed condition for a life in the open. Against this view, it 
should be pointed out that when it leaves the egg as a zoea, the young Pea Crab is well armed with 
three long and stout spines, and that the margins of his hind-body run into spiny projections, which 
are thrown off when he takes up residence in the shell of a mollusc. 
Another small point, though not without importance in this connection, is the fact that the 
male Pea Crab has a harder shell than the female, whose covering is really more like parchment 
than shell. The reason for the difference is this: the female, we believe, has never been found 
except in a bivalve shell; the male, who is more rarely seen than the female, has been taken at 
large. To seek the female he has to wander from mussel to mussel, and run the risks attending such 
adventures ; therefore, it is advisable that he should be clad in a stouter dress than the stay-at- 
home female. So his shell has not deteriorated to the same extent as that of his mate. 
It may be presumed that when the mussel opens the valves of its shell, and sets up the current 
that brings in the microscopic organisms upon which it feeds, there are larger particles flow in of 
which the mussel cannot make use, and which might by decaying in the shell set up a diseased 
condition. It is probable that the little Pea Crab takes all such as its share, and that by consuming 
it a benefit is conferred upon the mussel, which justifies it in giving house-room toits partner. In 
cases of true commensalism there is always some such reciprocal arrangement as the basis of 
partnership. 
The ancients variously regarded the Pea Crab as a burglar, who forces his way in when the valves 
are slightly open and eats up the mussel at leisure ; and as a friendly monitor, who by a timely nip 
indicated to the mussel when to close his doors with a snap, and so capture some curious visitor 
upon whom the confederates could banquet. 
There are many species of Pea Crab spread over the oceans of the world, and they have each 
some similar working arrangement with divers kinds of sea beasts, not necessarily molluscs. 
