Fig. [108].—Zoaea larva of Peneus, sp. × 25. A, A′, 1st and 2nd antennae; Ab.6, 6th abdominal appendage; Mxp, 2nd maxillipede; T, 4th–8th thoracic appendages (future walking legs). (After F. Müller.)
Some of the Peneid larvae take on very peculiar forms, e.g. the Zoaeae of the Sergestidae,[[134]] which often develop the most wonderful spines all over the body.
Fig. [109].—Mysis stage in the development of Peneus, sp. A.2, 2nd antenna; Ab.6, 6th abdominal appendage; T, telson; Th, the biramous thoracic appendages. (After Claus.)
The Caridea have a greatly abbreviated metamorphosis, the larva hatching out at a late Zoaea stage with all three pairs of maxillipedes fully formed and with a fully segmented abdomen. The succeeding thoracic limbs are added in order from before backwards, though the sixth pair of pleopods appear precociously as in the Peneidea. The other swimmerets do not begin to develop until the thoracic limbs are complete. Some Caridea show a yet more abbreviated metamorphosis, e.g. the fresh-water Palaemonetes varians of S. Europe, which hatches out at the Mysis stage.
We see, therefore, in the metamorphosis of the Macrura several apparently primitive features. In the first place, a free swimming Nauplius stage is preserved in certain forms, identical in all respects with the Nauplius of the Entomostraca. Secondly, the thoracic limbs when they are first developed are biramous, thus giving rise to the characteristic Mysis stage which links the Macrura on to the “Schizopoda.” Thirdly, the order of differentiation of the segments is typically from in front backwards, the only precociously developed appendage being the sixth abdominal. None of these characters are reproduced in the higher Decapoda in which there is never a free-living Nauplius, the first larval stage being the Zoaea; a number of the thoracic pereiopods, and usually all of them, are uniramous from the start; and the whole of the abdominal segments with their limbs tend to be precociously developed before the hinder thoracic segments make a distinct appearance.
Tribe 3. Peneidea.[[135]]
The third legs are chelate except in genera in which the legs are much reduced. The third maxillipedes are seven-jointed, the second maxillipedes have normal end-joints, and the first maxillipedes are without a lobe on the base of the exopodite. The pleura of the first abdominal segment are not overlapped by those of the second. The abdomen is without a sharp bend. The branchiae are usually not phyllobranchs.
Fam 1. Peneidae.—The last two pairs of legs are well developed, and there is a nearly complete series of gills. Cerataspis,[[136]] a pelagic form. Parapeneus, Peneus, Aristaeus, etc.
Fam. 2. Sergestidae.—The last or last two pairs of legs are reduced or lost. The gill-series is incomplete or wanting. Sergestes possesses gills, and the front end of the thorax is not greatly elongated. Lucifer has no gills, and the front of the thorax is greatly elongated, giving a very anomalous appearance to the animal. All the members of this family are pelagic in habit.