[Illustration: FIG. 15.--Nauplius of _Tetraclita porosa_ after the first moult.
Besides the nauplius and the zoea there are many other types of Crustacean larvae, distinguished by special names, though, as their occurrence is restricted within the limits of the smaller systematic groups, they are of less general interest. We need only mention the _Mysis_-stage (better termed Schizopod-stage) found in many Macrura (as, for example, the lobster), which differs from the adult in having large natatory exopodites on the thoracic legs.
Most of the larval forms swim freely at the surface of the sea, and many show special adaptations to this habit of life. As in many other "pelagic" organisms, spines and processes from the surface of the body are often developed, which are probably less important as defensive organs than as aids to flotation. This is well seen in the nauplius of many Cirripedia (fig. 15) and in nearly all zoeae. Perhaps the most striking example is the zoea-like larva of the _Sergestidae_, known as _Elaphocaris_, which has an extraordinary armature of ramified spines. The same purpose is probably served by the extreme flattening of the body in the membranous _Phyllosoma_-larva of the rock-lobsters and their allies (Loricata).
Although fossil remains of Crustacea are abundant, from the most ancient fossiliferous rocks down to the most recent, their study has hitherto contributed little to a precise knowledge of the phylogenetic history of the class. This is partly due to the fact that many important forms must have escaped fossilization altogether owing to their small size and delicate structure, while very many of those actually preserved are known only from the carapace or shell, the limbs being absent or represented only by indecipherable fragments. Further, many important groups were already differentiated when the geological record began. The Phyllopoda, Ostracoda and Cirripedia (Thyrostraca) are represented in Cambrian or Silurian rocks by forms which seem to have resembled closely those now existing, so that palaeontology can have little light to throw on the mode of origin of these groups. With the Malacostraca the case is little better. There is considerable reason for believing that the _Ceratiocaridae_, which are found from the Cambrian onwards, were allied to the existing _Nebalia_, and may possibly include the forerunners of the true Malacostraca, but nothing is definitely known of their appendages. In Palaeozoic formations, from the Upper Devonian onwards, numbers of shrimp-like forms are found which have been referred to the Schizopoda and the Decapoda, but here again the scanty information which may be gleaned as to the structure of the limbs rarely permits of definite conclusions as to their affinities. The recent discovery in the Tasmanian "schizopod" _Anaspides_, of what is believed to be a living representative of the Carboniferous and Permian _Syncarida_, has, however, afforded a clue to the affinities of some of these problematical forms.