United States or Yemen ? Vote for the TOP Country of the Week !
In this way the animal gradually approaches the Prawn-form, in which the median eye has become indistinct, the spine of the labrum, and the outer branches of the cheliferous and ambulatory feet have been lost, the mandibular palpi and the abdominal feet have acquired distinct joints and setae, and the branchiae come into action. The minute buds of the third pair of maxillipedes are visible.
The carapace possesses only a short, broad frontal process; the posterior margin of the tail is edged with numerous short setae. The tail bears five pairs of setae. The little animal usually holds itself extended straight in the water, with the head directed downwards. Between the large compound eyes there is in them a small median eye.
The crab then selects choice bits of weed from its old shell and fastens them to itself by the setae, which soon curl at the tips like the tendrils of a vine, and so hold them firmly.
Notwithstanding their agreement in this important peculiarity, the Nauplii of these two orders present material differences in many other particulars. The young Cirripedes have a mouth, stomach, intestine, and anus, and their two posterior pairs of limbs are beset with multifarious teeth, setae, and hooks, which certainly assist in the inception of nourishment.
When these crabs shed their shells, which they must do periodically to allow of growth, they retire to a dark corner and draw themselves out of a slit between the back and the abdomen, legs and all, which must, I imagine, be a delicate and somewhat painful proceeding. After emerging, they are, of course, quite soft, and the setae on the carapace and legs are flexible.
The small cephalic lobe bore four eyes and five tentacles; each body-segment had on each side at the margin a tuft of simple setae directed obliquely upwards, and at some distance from this, upon the ventral surface, a group of thicker setae with a strongly uncinate bidentate apex.
The larva still continues like a Nauplius, as the three anterior pairs of limbs represent rowing feet; at the next moult it is converted into the youngest Cyclops-like state, when it resembles the adult animal in the structure of the antennae and buccal organs, although the number of limbs and body segments is still much less, for only the rudiments of the third and fourth pairs of natatory feet have made their appearance in the form of cushions fringed with setae, and the body consists of the oval cephalothorax, the second, third, and fourth thoracic segments, and an elongated terminal joint.
The antennae, organs of the mouth, and natatory feet, exhibit the same structure. But the tail bears FIVE pairs of setae, and the dorsal spine is wanting, whilst, on the contrary, the frontal process and the lateral spines are of extraordinary length, and directed straight forward and backward. Zoea of Porcellana stellicola, F. Mull. Magnified 15 diam.
The simple median eye appears earlier, and would therefore be more important than the compound paired eyes; the scale of the antennae in the Prawns would be more important than the flagellum; the maxillipedes of the Decapoda would be more important than the chelae and ambulatory feet, and the anterior six pairs of feet in the Isopoda, than the precisely similarly formed seventh pair; in the Amphipoda the most important of all organs would be the "micropylar apparatus," which disappears without leaving a trace soon after hatching; in Cyclops the setae of the tail would be more important than all the natatory feet; in the Cirripedia the posterior antennae, as to which we do not know what becomes of them, would be more important than the cirri, and so forth.
The outer antennae exhibit a well-developed and sometimes distinctly articulated scale, and within this usually a spiniform process; the flagellum appears generally to be still wanting. The third pair of maxillipedes seems to be always present, at least in the form of considerable rudiments. The spatuliform caudal lamina bears from five to six pairs of setae on its hinder margin.
Word Of The Day