The principal difference from the _Astacina_ is exhibited by the gills, of which there are twenty on each side; namely, six podobranchiæ, ten arthrobranchiæ, and four {259} fully developed pleurobranchiæ. Moreover, the branchial filaments of these gills are much stiffer and more closely set than in most crayfishes. But the most important distinction is presented by the podobranchiæ, in which the stem is, as it were, completely split into two parts longitudinally (as in fig. 68, B); one half (_ep_) corresponding with the lamina of the crayfish gill, and the other (_pl_) with its plume. Hence the base (_b_) of the podobranchia bears the gill in front; while, behind, it is continued into a broad epipoditic plate (_ep_) slightly folded upon itself longitudinally but not plaited, as in the crayfish. [Illustration: FIG. 68. Podobranchiæ of A, _Parastacus_; B, _Nephrops_; C, _Palæmon_. A′, C′, transverse sections of A and C respectively. _a_, point of attachment; _al_, wing-like expansion of the stem; _b_, base; _br_, branchial filaments; _ep_, epipodite; _l_, branchial laminæ; _pl_, plume; _st_, stem.] [Illustration: FIG. 69. _Nephrops norvegicus_ (1/2 nat. size).] The Norway Lobster (_Nephrops norvegicus_, fig. 69) {261} resembles the lobster in those respects in which the latter differs from the crayfishes: but the antennary squame is large; and, in addition, the branchial plume of the podobranchia of the second maxillipede is very small or absent, so that the total number of functional branchiæ is reduced to nineteen on each side. These two genera, _Homarus_ and _Nephrops_, therefore, represent a family, _Homarina_, constructed upon the same common plan as the crayfishes, but differing so far from the _Astacina_ in the structure of the branchiæ and in some other points, that the distinction must be expressed by putting them into a different tribe. It is obvious that the special characteristics of the plan of the _Homarina_ give it much more likeness to that of the _Potamobiidæ_ than to that of the _Parastacidæ_. The Rock Lobster (_Palinurus_, fig. 70) differs much more from the crayfishes than either the common lobster or the Norway lobster does. Thus, to refer only to the more important distinctions, the antennæ are enormous; none of the five posterior pairs of thoracic limbs are chelate, and the first pair are not so large in proportion to the rest as in the crayfishes and lobsters. The posterior thoracic sterna are very broad, not comparatively narrow, as in the foregoing genera. There are no appendages to the first somite of the abdomen in either sex. In this respect, it is curious to observe that, in contradistinction from the _Homarina_, the Rock Lobsters are more closely allied to the _Parastacidæ_ than to the _Potamobiidæ_. {263} The gills are similar to those of the lobsters, but reach the number of twenty-one on each side. [Illustration: FIG. 70. _Palinurus vulgaris_ (about 1/4 nat. size).]