Orders: Ostracoda, Phyllopoda and Trilobitae.
Orders: Branchiopoda (Sub-orders: Phyllopoda, Cladocera, Branchiura), Ostracoda, Copepoda.
There, as above explained, Leach began the subdivision of Muller's too comprehensive genus, the result being that Lynceus belongs to the Phyllopoda, and Chydorus (Leach, 1816) properly gives its name to the present family, in which the doubly convoluted intestine is so remarkable.
Sometimes they are applied, as in the Copepoda, to the limb-bearing and limbless regions of the trunk, while in other cases, as in the Phyllopoda, they denote, respectively, the regions in front of and behind the genital apertures.
- Phyllopoda and Phyllocarida.
In all the Phyllopoda the number of endites is six, and the proximal one is more or less distinctly specialized as a gnathobase, working against its fellow of the opposite side in seizing food and transferring it to the mouth.
The Phyllopoda are the only Crustacea in which distinct and functional gnathobasic processes are found on appendages far removed from the mouth.
It is not altogether easy to recognize the homologies of the endites and exites even within the order Phyllopoda, and the identification of the two distal endites as corresponding to the endopodite and exopodite of higher Crustacea is not free from difficulty.
In many Entomostraca (Phyllopoda, Cladocera, Ostracoda, Copepoda) they are important, and sometimes the only, organs of locomotion.
In some male Phyllopoda they form complex " claspers " for holding the female.
In the Phyllopoda they are for the most part all alike, though one or two of the anterior pairs may be specialized as sensory (Apus) or grasping (Estheriidae) organs.
In the primitive Malacostraca the gills were probably, as in the Phyllopoda and in Nebalia, the modified epipodites of the thoracic limbs, and this is the condition found in some Schizopoda.
In a few Entomostraca (some Phyllopoda and Ostracoda) the chitinous lining of the fore-gut develops spines and hairs which help to triturate and strain the food, and among the Ostracods there is occasionally (Bairdia) a more elaborate armature of toothed plates moved by muscles.
Thus, in the Phyllopoda, the antennal gland develops early and is functional during a great part of the larval life, but it ultimately atrophies, and in the adult (as in most Entomostraca) the maxillary gland is the functional excretory organ.
In the primitive Phyllopoda the ventral chain retains the ladder-like arrangement found in some Annelids and lower worms, the two halves being widely separated and the pairs of ganglia connected together across the middle line by double transverse commissures.
In the Phyllopoda it consists mainly of two pairs of ganglionic centres, giving origin respectively to the optic and antennular nerves.
Apart from certain doubtful and possibly abnormal instances among Phyllopoda and Amphipoda, the only exceptions are the sessile Cirripedia and some parasitic Isopoda (Cymothoidae), where hermaphroditism is the rule.
(After Morse.) position is occupied by the genital apertures of certain Phyllopoda (Polyartemia), which lie behind the nineteenth trunk-somite.
Very few Crustacea are viviparous in the sense that the eggs are retained within the body until hatching takes place (some Phyllopoda), but, on the other hand, the great majority carry the eggs in some way or other after their extrusion.
In some Phyllopoda (A pus) egg-sacs are formed by modification of certain of the thoracic feet.
The eggs are retained between the valves of the shell in some Phyllopoda and in the Cladocera and Ostracoda, and they lie in the mantle cavity in the Cirripedia.
A nauplius larva differing only in details from the typical form just described is found in the majority of the Phyllopoda, Copepoda and Cirripedia, and in a more modified form, in some Ostracoda.
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.
It is probable also, as already mentioned, that the leaf-like appendages of the Phyllopoda are of a primitive type, and attempts have been made to refer their structure to that of the Annelid parapodium.
In many respects, however, the Phyllopoda, and especially A pus, have diverged considerably from the primitive Crustacean type.
Whether any of the obscure fossils generally referred to the Phyllopoda or Phyllocarida may have approximated to this hypothetical form it is impossible to say.
The Phyllopoda must have branched off very early and from them to the Cladocera the way is clear.