VII. 0 (ep.) 0 0 0 = 0 VIII. 1 1 0 0 = 2 IX. 1 1 1 0 = 3 X. 0 1 1 0 = 2 XI. 0 0 0 1 = 1 XII. 0 0 0 1 = 1 XIII. 0 0 0 0 = 0 XIV. 0 0 0 0 = 0 ── ── ── ── ── 2 + ep. + 3 + 2 + 2 = 9 + ep. It will be observed that the suppression of branchiæ has here taken place in all the series, and at both the anterior and the posterior ends of each. But the defect in total number is made up by the increase of size, not of the pleurobranchiæ alone, as in the case of the prawns, but of the arthrobranchiæ as well. At the same time the whole apparatus has become more specialized and perfected as a breathing organ. The close fitting of the edges of the carapace, and the possibility of closing the inhalent and exhalent apertures, render the crabs much more independent of actual immersion in water than most of their congeners; and some of them habitually live on dry land and breathe by means of the atmospheric air which they take into and expel from their branchial cavities. Notwithstanding all these wide departures from the structure and habits of the crayfishes, however, attentive examination shows that the plan of construction of the {278} crab is, in all fundamental respects, the same as that of the crayfish. The body is made up of the same number of somites. The appendages of the head and of the thorax are identical in number, in function, and even in the general pattern of their structure. But two pairs of abdominal appendages in the female, and four pairs in the male, have disappeared. The exopodites of the antennæ have vanished, and not even epipodites remain to represent the podobranchiæ of the posterior five pairs of thoracic limbs. The exceedingly elongated eye-stalks are turned backwards and outwards, above the bases of the antennules and the antennæ, and the bases of the latter have become united with the edges of the carapace in front of them. In this manner the extraordinary face, or _metope_ (fig. 72, B) of the crab results from a simple modification of the arrangement of parts, every one of which exists in the crayfish. The same common plan serves for both. * * * * *