The size of the animals varies greatly, from forms a few millimetres in length to Gigantorhynchus gigas, which measures from 10 to 65 cms. The adults live in great numbers in the alimentary canal of some vertebrate, usually fish, the larvae are as a rule encysted in the body cavity of some invertebrate, most often an insect or crustacean, more rarely a small fish.
It is then, if lucky, eaten by some crustacean, or insect, more rarely by a fish.
The most aberrant type of larva is that of the genus Prosopistoma, which was originally described as an entomostracous crustacean on account of the presence of a large carapace overlapping the greater part of the body.
The central nervous systems of Limulus and of Scorpio present closer agreement in structure than can be found when a Crustacean is compared with either.
(After Lankester, loc. cit.) eyes, it is to be noted that no Crustacean has structures corresponding to the peculiar diplostichous central eyes, though these occur again (with differences in detail) in Hexapoda.
The latter open at the base of the fifth pair of limbs of the Crustacean, just as the coxal glands open on the coxal joint of the fifth pair of limbs of the Arachnid.
A minute entosternite having the above-described structure is found in the Crustacean Apus between the bases of the mandibles, and also in the Decapoda in a similar position, but in no Crustacean does it attain to any size or importance.
Not only are the blood corpuscles of Limulus more like in form and granulation to those of Scorpio than to those of any Crustacean, but the fluid is in both animals strongly impregnated with the blue-coloured respiratory proteid, haemocyanin.
It has been considered by them as proving that Limulus, in spite of all its special agreements with Scorpio (which, however, have scarcely been appreciated by the writers in question), really belongs to the Crustacean line of descent, whilst Scorpio, by possessing Malpighian tubes, is declared to be unmistakably tied together with the other Arachnida to the tracheate Arthropods, the Hexapods, Diplopods, and Chilopods, which all possess Malpighian tubes.
On the other hand, the land crabs are at an immense distance from these simple forms. The record of the Crustacean familytree is, in fact, a fairly complete E .Ps one - the lower primitive members s: of the group are still represented c' by living forms in great abundance.
No Crustacean exhibits this trilobite form.
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.
The reduction of the organism to seven leg-bearing somites, of which the first pair, as in so many Eu-arachnida, are chelate, is a form of degeneration connected with a peculiar quasi-parasitic habit resembling that of the crustacean Laemodipoda.
The cercaria swims freely for a time and either encysts directly on grass or weeds or it enters a second host which may be another mollusc, an insect, crustacean or fish, and then encysts.
The equivalent terms applied by Sars are Anostraca, Notostraca, Conchostraca, involving a termination already appropriated to higher divisions of the Crustacean class, for which it ought to be reserved.
Similarly, early forms of the crustacean sub-class Merostomata have been traced to the pre-Cambrian of North America.
All known species are parasitic on the Crustacean Nebalia; Seison Claus; Paraseison Plate.
Drilophagus and Albertia are parasitic on the surface or within the gut of Naid Oligochaete worms: Seisonaceae are ectoparasitic on the Crustacean Nebalia, Proales werneckii forms galls within the Conferva Vaucheria, and P. parasita infests the central jelly of the Phytoflagellate Volvox; P. petromyzon is a frequent commensal in the gill cavity of some Cladoceran Crustacean Eurycereus lamellatus.
The life of the land waters was also noteworthy, especially for the great deployment of what may be called the crustacean-ostracodermo-vertebrate group. The crustacea were represented by eurypterids, the ostracoderms by numerous strange, vertebrate-like forms (Cephalaspis, Gyathaspis, Trematopsis, Bothriolepss, &c.), and the vertebrates by a great variety of fishes, The land life of the period is represented more fully among the fossils than that of any preceding period.
Lepton; commensal with the Crustacean Gebia; British.
The order thus sharply divided is united by an intimate interlacing of characters, and forms a compact whole at present defying intrusion from any other crustacean group. Since 1775, when J.
In certain parasites, for instance, the adults have lost every trace not only of Crustacean but even of Arthropodous structure, and the only clue to their zoological position is that afforded by the study of their development.
In no Crustacean, however, do all the somites of the body remain distinct.
Somite corresponding to the eyes (the morphological nature of which is discussed below), the smallest number of head-somites so united in any Crustacean is five.
The heart was tubular in form, 3a and 3b, Two crustacean teeth extending the whole length of the (of Dithyrocaris) from the body, and having a pair of ostia Carboniferous series of in each somite.
It seems certain, therefore, that the possession of a nauplius larva must be regarded as a very primitive character of the Crustacean stock.
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.
The opinion now most generally held is that the primitive Crustacean type is most nearly approached by certain Phyllopods such as Apus.
In many respects, however, the Phyllopoda, and especially A pus, have diverged considerably from the primitive Crustacean type.
In order to reconstruct the hypothetical ancestral Crustacean, therefore, it is necessary to combine the characters of several of the existing groups.
It is to be noted, however, that the Trilobita, which, according to the classification here adopted, are dealt with under Arachnida, are not very far removed, except in such characters as the absence of a shellfold and of eye-stalks, from the primitive Crustacean here sketched.
It is an ancestral larval form, corresponding perhaps to the stages immediately succeeding the trochophore in the development of Annelids, but with some of the later-acquired Crustacean characters superposed upon it.
It was not till Vaughan Thompson demonstrated, in 1830, their development from a free-swimming and typically Crustacean larva that it came to be recognized that, in Huxley's graphic phrase, "a barnacle may be said to be a Crustacean fixed by its head and kicking the food into its mouth with its legs."