the suggestion arises that each hypothetically complete thoracic somite should possess four gills on each side, giving the following

Hypothetically complete branchial formula.

Starting from this hypothetically complete branchial formula, we may regard all the actual formulæ as produced from it by the more or less complete suppression of the most anterior, or of the most posterior branchiæ, or of both, in each series. In the case of the podobranchiæ, the branchiæ are converted into epipodites; in that of the other branchiæ, they become rudimentary, or disappear.

In general appearance a common prawn (Palemon, fig. 71) is very similar to a miniature lobster or crayfish. Nor does a closer examination fail to reveal a complete fundamental likeness. The number of the somites, and of the appendages, and their general character and dispo

sition, are in fact the same. But, in the prawn, the abdomen is much larger in proportion to the cephalothorax; the

basal scale, or expodite of the antenna, is much larger; the external maxillipedes are longer, and differ less from the succeeding thoracic appendages. The first pair of these, which answers to the forceps of the crayfish, is chelate, but it is very slender; the second pair, also chelate, is always larger than the first, and is sometimes exceedingly

long and strong (fig. 71, B); the remaining thoracic limbs are terminated by simple claws. The five anterior abdominal somites are all provided with large swimmerets, which are used like paddles, when the animal swims quietly; and, in the males, the first pair is only slightly different from the rest. The rostrum is very large, and strongly serrated.

None of these differences from the crayfish, however, is so great, as to prepare us for the remarkable change observable in the respiratory organs. The total number of the gills is only eight. Of these, five are large pleurobranchia, attached to the epimera of the five hinder thoracic somites; two are arthrobranchiæ, fixed to the interarticular membrane of the external maxillipede; and one, which is the only complete podobranchia, belongs to the second maxillipede. The podobranchiæ of the first and third maxillipedes are represented only by small epipodites. The branchial formula therefore is :

PHYLLOBRANCHIÆ.

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The prawn, in fact, presents us with an extreme case of that kind of modification of the branchial system, of which Penæus has furnished a less complete example. i The series of the podobranchiæ is reduced almost to nothing, while the large pleurobranchiæ are the chief organs of respiration.

But this is not the only difference. The prawn's gills are not brush-like, but are foliaceous. They are not trichobranchia, but phyllobranchiæ; that is to say, the central stem of the branchia, instead of being beset with numerous series of slender filaments, bears only two rows of broad flat lamella (fig. 68, C, C', ), which are attached to opposite sides of the stem (C', s), and gradually diminish in size from the region of the stem by which it is fixed, upwards and downwards. These lamellæ are superimposed closely upon one another, like the leaves of a book; and the blood traversing the numerous passages by which their substance is excavated, comes into close relation with the currents of aerated water, which are driven between the branchial leaflets by a respiratory mechanism of the same nature as that of the crayfish.

Different as these phyllobranchia of the prawns are in appearance from the trichobranchiæ of the preceding Crustacea, they are easily reduced to the same type. For in the genus Axius, which is closely allied to the lobsters, each branchial stem bears a single series of filaments on its opposite sides; and if these biserial filaments are supposed to widen out into broad leaflets, the transition from

the trichobranchia to the phyllobranchia will be very easily effected.

The shrimp (Crangon) also possesses phyllobranchiæ, and differs from the prawn chiefly in the character of its locomotive and prehensile thoracic limbs.

There are yet other very well-known marine animals, which, in common appreciation, are always associated with the lobsters and crayfishes, although the difference of general appearance is vastly greater than in any of the cases which have yet been considered. These are the

Crabs.

In all the forms we have hitherto been considering, the abdomen is as long as, or longer than, the cephalothorax, while its width is the same, or but little less. The sixth somite has very large appendages, which, together with the telson, make up a powerful tail-fin; and the large abdomen is thus fitted for playing an important part in locomotion.

Again, the length of the cephalothorax is much greater than its width, and it is produced in front into a long rostrum. The bases of the antennæ are freely movable, and they are provided with a movable exopodite. Moreover, the eye-stalks are not inclosed in a cavity or orbit, and the eyes themselves appear above and in front of the antennules. The external maxillipedes are narrow, and their endopodites are more or less leg-like.

None of these statements apply to the crabs. In these