| Somites and their Appendages. | Podobranchiæ. | Arthrobranchiæ. | Pleurobranchiæ. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Anterior. | Posterior. | ||||||||
| 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.
FIG. 71. Palæmon jamaicensis (about 5⁄7 nat. size). A, female; B, fifth thoracic appendage of male.
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 pleurobranchiæ, 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:—
| Somites and their Appendages. | Podobranchiæ. | Arthrobranchiæ. | Pleurobranchiæ. | ||||||
|---|---|---|---|---|---|---|---|---|---|
| Anterior. | Posterior. | ||||||||
| VII. | 0 (ep.) | 0 | 0 | 0 | = | 0 (ep.) | |||
| VIII. | 1 | 0 | 0 | 0 | = | 1 | |||
| IX. | 0 (ep.) | 1 | 1 | 0 | = | 2 (ep.) | |||
| X. | 0 | 0 | 0 | 1 | = | 1 | |||
| XI. | 0 | 0 | 0 | 1 | = | 1 | |||
| XII. | 0 | 0 | 0 | 1 | = | 1 | |||
| XIII. | 0 | 0 | 0 | 1 | = | 1 | |||
| XIV. | 0 | 0 | 0 | 1 | = | 1 | |||
| 1 + 2 ep. | + | 1 | + | 1 | + | 5 | = | 8 + 2 ep. | |
{271}
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. The series of the podobranchiæ is reduced almost to nothing, while the large pleurobranchiæ are the chief organs of respiration.