LARVAL FORM OF CRUSTACEA
Larval form of crustacea
Several larval forms are met within Crustacea and special terms are to each one of them, according to the group in which they occur. The following important larval forms among Crustacea.
1-Nauplius-It is the simplest, commonest and earliest larval form in Crustacea.
2-Metanauplius-The metanauplius, which follows the nauplius, is a vaguely defined stage. It consists of an oval cephalothorax, an elongated trunk-region and abdoman terminating in a caudal fork provided with setae.
3-Cypris-In some Cirripedia the nauplius passes into the cypris stage, in which the body and the appendages are enclosed within a bivalved carapace with an adductor muscle to close it.
4-Protozoaea-In Penaeus, the metanauplius is succeeded by the protozoa stage with 7 pairs of appendages and the begining of segmenation.
5-Zoaea-Zoaea is the second important larva of the Crustacea, the first being the nauplius.Protozoaea stage is succeeded by the zoaea stage..
6-Metazoaea-The older zoaea or metazoaea has well formed third maxillipedes, which are biramous and swimming organs in Anomura, but uniramous and non-swimming in Brachyura.
7-Calyptopsis-In Euphausiacea, one of the larval stages is termed calyptopsis. It is similar in all respects to a typical zoaea except that the paired eyes are not stalked but sessile.
9-Alima-The so-called alima larva of Squilla which hatches out from the egg directly, is a modified zoaea. It is a pelagic larva, having a glass-like transparency and occurring in large numbers in the plankton.
8-Erichthus-In stomatopoda, two main types of larvae are found, erichthus and alima.
In erihthus larva, a carapace covers the greater part of the body.
10-Megalopa- In true crab, the zoaea passes through successive moults into the postlarval megalopa stage. It has a broad and crab like unsegmented cephalothorax.
11-Glaucothoe- In hermit crabs, the metazoaea leads to the glaucothoe stage. It corresponds to the megalopa stage of Brachyura with a large symmetrical abdomen and full complement of adult appendages.
12-Mysis- In Penaeus and Thalassinidea among the Anomura, the zoaea, instead of converting into the megalopa stage,mouths into the postlarval mysis or schizopod resembles the adult Mysis.
13-Phyllosoma- In the rock-lobster, the newly hatched larva, called the Phyllosoma or glass-crab, is a greatly modified mysis stage. A narrow constriction demarcates the head from thorax.
Significance of larval forms The development of certain Crustacea is often used to illustrate the principle of recapitulation which Haeckel phrased as the biogenetic law. In their development after hatching they go through a series of swimming larval stages with increasing complexity until the adult condition is reached.
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