Entomos´traca, a sub-class of crustacea, including forms which are mostly small, and comprising four orders: (1) Branchiopoda, brine-shrimps and water-fleas; (2) Copepoda, including the freshwater Cyclops, and numerous marine
species contributing to the floating surface population (plankton); (3) Cirripedia, barnacles; (4) Ostracoda, types such as Cypris enclosed in a bivalve shell.
1, Cœnūrus cerebrālis (producing the staggers in sheep). a, Heads (shown on the surface) separately.
2, Cysticercus cellulosæ (causing the measles in pigs). b, Head.
Entozo´a, a general name for the various parasitic worms that infest the bodies of other animals. Some are found in the intestines, others in the liver, brain, muscles, and other tissues. They pass through different stages in their development, and at each stage may occupy a different organ (or tissue), and usually a different animal. Thus the cystic or bladder-worm, whose presence in the brain of sheep causes staggers, is the immature form of a tapeworm of the dog, &c. The number of species is being reduced as the relations of the different forms are studied. They belong to the two phyla Platyhelminthes (flat-worms) and Nemathelminthes (thread-worms, &c.). The former embraces flukes (Trematoda) and tapeworms (Cestoda); while the latter includes thread-worms and round-worms (Nematoda), and spiny-headed worms (Acanthocephala).
Entrenchments. The employment of entrenchments, or earthworks, in connection with military operations dates from the earliest times. Entrenched camps, made by the Romans, are still to be seen in many parts of the country. The primary object, in early designs, was to offer a material obstacle to the assaulting enemy; the defenders manned a high parapet overlooking a formidable ditch, and a hand-to-hand conflict decided the issue. The development of fire-arms, and especially of artillery, depreciated the value of entrenchments as affording material obstacles, but gave them a gradually increasing value as a means of protecting the defenders from missiles. The provision of cover, that is to say, concealment from view or protection from fire, ultimately became the dominant factor in the design of earthworks. Obstacles were still necessary, but they had to be provided by other means. Successive improvements in firearms altered the nature of the cover which it was necessary and practicable to provide. A bullet-proof parapet has always been an essential feature, but whereas a few inches of earth sufficed to stop a musket-ball, the modern rifle-bullet will penetrate a thickness of nearly four feet. The introduction, during last century, of shrapnel shell, the bullets from which descend at a steep angle, was followed by the adoption of overhead cover as a standard feature in trench design. This persisted for several years, until the great increase in the power of high-explosive shell made it impossible to construct any form of roof which would withstand bombardment, even by the lighter artillery accompanying an army in the field, and still permit of the defenders using their rifles from below it. Modern fire-positions are made open, i.e. without overhead cover.
All past wars have proved that victory can only be won as the result of offensive action. Nevertheless, in any campaign it will not be possible to attack at all times and in all places. The provision of the strongest possible force at the vital point necessitates a defensive attitude on other parts of the front. Although entrenchments presuppose a defensive attitude locally, they play an important part in offensive operations. The ultimate aim is that, by a skilful use of entrenchments, a commander may be able to reduce to a minimum the strength of his force in actual combat with the enemy, and thus to retain at his disposal a reserve of troops for offensive action.
Entrenchments may be either hasty or deliberate. Hasty entrenchments are those made on the actual field of battle: by the attackers, to secure the ground won prior to another bound forward; by the defenders, to hold up the attack pending fresh dispositions of troops in rear. The amount of digging that can be done is necessarily small; existing cover must be utilized to the utmost. This may consist of ditches, hedges, sunken roads, railway embankments and cuttings, buildings, woods, shell-holes, &c. All of these are readily convertible into strong defences. The test of battle has proved, over and over again, that troops well trained in adapting natural cover to defence are very difficult to dislodge, once they have dug themselves in. Deliberate entrenchments are employed in the gradual building up of a trench system when once the opportunity of manœuvre has ceased to exist; or in the preparation of a defensive position somewhat remote from the scene of active operations.
The following are the salient features in the design of modern entrenchments: (1) A parapet 18 inches high, and upwards of 5 feet thick, in front of every fire-trench. (2) Longitudinal division of every trench, either by projecting buttresses of earth or by bends, so that no straight portion exceeds 10 yards in length. The effect of this is to give protection against