This case is very interesting; for it shows that the whole of the podobranchiæ may lose their branchial character, and be reduced to epipodites, as is the case with the first in the crayfish and lobster, and indeed in most of the forms under consideration. And since all but one of the somites bear both arthrobranchiæ and pleurobranchiæ, {268} the suggestion arises that each hypothetically complete thoracic somite should possess four gills on each side, giving the following _Hypothetically complete branchial formula._ Somites and Arthrobranchiæ. their Podobranchiæ. /───────^───────\ Pleuro- Appendages. Anterior. Posterior. branchiæ. VII. 1 1 1 1 = 4 VIII. 1 1 1 1 = 4 IX. 1 1 1 1 = 4 X. 1 1 1 1 = 4 XI. 1 1 1 1 = 4 XII. 1 1 1 1 = 4 XIII. 1 1 1 1 = 4 XIV. 1 1 1 1 = 4 ── ── ── ── ── 8 + 8 + 8 + 8 = 32 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 (_Palæmon_, 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 {269} disposition, 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 {270} 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.