38), like that of Limulus and Scorpio, and that lateral spines on the pleura of the somites are frequent as in Limulus, and that neither metasomatic fusion of somites nor post-anal spine, nor lateral pleural spines are found in any Crustacean, nor all three together in any Arthropod besides the trilobites and Limulus - the claim of the trilobites to be considered as representing one order of a lower grade of Arachnida, comparable to the grade Entomostraca of the Crustacea, seems to be established.
Orders: (a) Malacostraca: Decapoda, Stomapoda, Amphipoda, Laemodipoda, Isopoda; (b) Entomostraca: Branchioloda, Poecilopoda, Trilobitae.
Classes: Arachnida, Insecta (including Sub-Classes M y riapoda, Hexapoda), Crustacea (including Sub-Classes Entomostraca, Malacostraca), Epizoa (Epizootic Crustacea), Annellata (Chaeto p ods and Leeches), Cirripedia.
Orders: Entomostraca and Malacostraca.
Under the heading Crustacea the Entomostraca have already been distinguished not only from the Thyrostraca or Cirripedes, but also from the Malacostraca, and an intermediate group of which the true position is still disputed.
For the study of freshwater Entomostraca large possibilities are now opened to the naturalist.
Those, too, who send or bring the foreign soil should exercise a little thought in the choice of it, since dry earth that has never had any Entomostraca near it at home will not become fertile in them by the mere fact of exportation.
Apus australiensis (Spencer and Hall, 1896) may rank as the largest of the Entomostraca, reaching in the male, from front of shield to end of telson, a length of 70 mm., in the female of 64 mm.
Scott, "Entomostraca from the Gulf of Guinea," Trans.
St Petersburg, " Caspian Entomostraca " (1897); Giesbrecht and Schmeil, " Copepoda gymnoplea," Das Tierreich (1898); Giesbrecht, " Asterocheriden," F.
The thin-walled eggs are often termed "summer-eggs," the fertilized ones "winter" or "ephippial" eggs (by parity with the phyllopod Entomostraca, q.v.).
Besides the sedentary Cirripedia, numbers of the smaller forms, especially among the Entomostraca, subsist on floating particles of organic matter swept within reach of the jaws by the movements of the other limbs.
In the various groups of the Entomostraca, on the other hand, the terms thorax and abdomen, though conveniently employed for purposes of systematic description, do not imply any homology with the regions so named in the Malacostraca.
In many Entomostraca (Phyllopoda, Cladocera, Ostracoda, Copepoda) they are important, and sometimes the only, organs of locomotion.
In other Entomostraca considerable differentiation may take place, but the series is never divided into definite " tagmata " or groups of similarly modified appendages.
In many of the smaller Entomostraca (Copepoda and most Ostracoda) no special gills are present, and respiration is carried on by the general surface of the body and limbs.
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.
In many Entomostraca the heart is absent, and it is impossible to speak of a " circulation " in the proper sense of the term, the blood being merely driven hither and thither by the movements of the body and limbs and of the alimentary canal.
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.
The former is generally present in the earliest larval stages (nauplius), and in some Entomostraca (e.g.
In the Malacostraca the sessile eyed groups are certainly less primitive than some of those with stalked eyes, and among the Entomostraca also there is some evidence pointing in the same direction.
Latreille, who, in the beginning of the 19th century, divided the class into Entomostraca and Malacostraca.
The latter division, characterized by the possession of 19 somites and pairs of appendages (apart from the eyes), by the division of the appendages into two tagmata corresponding to cephalothorax and abdomen, and by the constancy in position of the generative apertures, differing in the two sexes, is unquestionably a natural group. The Entomostraca, however, are certainly a heterogeneous assemblage, defined only by negative characters, and the name is retained only for the sake of convenience, just as it is often useful to speak of a still more heterogeneous and unnatural assemblage of animals as Invertebrata.
The remaining groups are dealt with under the headings Entomostraca and Malacostraca, the annectent group Leptostraca being included in the former.