where the whole crop is strengthened by a framework of bars, the
incudate mastax lies in a little postero-ventral pouch which can be
everted through the crop and mouth. The stomach is generally
large; its wall consists of a layer of very large ciliated cells, which
often contain fat globules and yellowish-green or brown particles,
and outside these a connective tissue membrane; muscular fibrillate
have also been described. Very constantly a pair of simple sack-like
glands open into the stomach, and probably represent the
hepato-pancreatic glands of other Invertebrates.
Following upon the stomach there is a longer or shorter intestine, which ends in the cloaca. The intestine is lined by ciliated cells. In forms living in a tube the intestine turns round and runs forward, the cloaca being placed so as to debouch over the margin of the tube. The cloaca is often very large; the nephridia and oviducts may open into it, and the eggs lodge there on their way outwards; they are thrown out, as are the faecal masses, by an eversion of the cloaca. Asplanchna, Notommata seiboldii, and certain species of Ascomorpha are devoid of intestine or anus, excrementitious matters being ejected through the mouth. The body cavity (archicoele) contains a fluid in which very minute corpuscles have been detected. There is no trace of a true vascular system. The nephridia (fig. 1, B, n) present a very interesting stage of development. They consist of a pair of tubules with an intracellular lumen running up the sides of the body, at times merely sinuous, at others considerably convoluted. From these are given off at irregular intervals short lateral branches, each of which terminates in a flame-cell (f) precisely similar in structure to the flame-cells found in Planarians, Trematodes and Cestodes; here as there the question whether they are open to the body cavity or not must probably be answered in the negative. At the base these tubes open either into a permanent bladder (fig. 1, bl) which communicates with the cloaca, or directly into the cloaca. They have the same functions as the contractile vacuole of freshwater Protozoa (q.v.).
Nervous System.—There is a large ganglion lying in close contact with the pharynx, proximal to the crop and on its antero-dorsal side; in Bdelloidaceae at least it is united by short connectives with a smaller postero-ventral ganglion to form a nerve collar. From this simple nerve fibres are given off to the body-wall, especially to the ciliated cells of the corona, to the foot, and also to the muscles and sense organs.
 |
|
Fig. 5.—Pedalion mira. A, lateral surface view of an adult female: a, median ventral appendage; b, median dorsal appendage; c, distal ventro-lateral appendage; d, dorso-lateral appendage; f, dorsal antenna; g, “chin”; x′, cephalotroch. B, lateral view, showing viscera: oc, eye-spots; n, nephridia; e, ciliated toes; other letters as above. C, ventral view: x′, trochus; x, cingulum; other letters as above. D, ventral view, showing the musculature (cf. text). E, dorsal view of a male: a, lateral appendages; b, dorsal appendage. F, lateral view of a male. G, enlarged view of the antenna f. H, enlarged view of the median ventral appendage. (All after Hudson.) |
The sense organs are eyes, antennae, sensory styles and a statocyst in a few species. The eyes are refractive globules set in a cup of red pigment traversed by a nerve fibre, and lie on the proximal side of the body, directly on the postero-dorsal surface of the brain, or at a little distance from it, on the neck, often within the circle on the corona, and usually well within the transparent body. There may be one, a pair, or rarely more, the outer ones being more or less rudimentary. The antennae are short tubular extensions of the body wall, sometimes retractile with a depressed tip from which protrudes a tuft of fine stiff bristles. They are possibly organs of external taste (smell) as well as of touch. Typically there are two pairs-a proximal, more or less approximated on the postero-dorsal surface, and a distal pair, more widely separate. But the proximal pair are often fused into a single median antenna (supplied, however, by two nerves), and in one case at least the distal pair may be similarly fused. Additional paired antennae may occur within the coronal surface, which is the seat of the sensory styles, of less complex structure, which occur in many genera. The statocyst (retro-cerebral organ of P. Marius de Beauchamp) is a sac filled with highly refractive granules soluble in dilute acids, and opening by a slender duct (or a pair) to the surface: its function is doubtless that of an organ of equilibrium, and it resembles in its opening to the surface the primitive internal ear of even Vertebrates, for the duct to the surface persists through life in the sharks.
|
|
Fig. 6.—Male Rotifers. 1, Euchlanis deflexa; 2, lateral; 2a dorsal views of Colurus bicuspidatus; 3, Notops brachionus; 4, Diglena permollis; 5, Gastropus minor; 6, Anuraea serrata; 7, Ascomorpha parasita; 8, Notholca heptodon. (Drawn from specimens by F. R. Dixon Nuttall.) |
Locomotor Organs.—Most free rotifers swim by the corona, aided by the ciliated auricles when present. In Bdelloidaceae this may aternate with a leech-like gait; the corona being withdrawn, the cupped end of the proboscis serves as a sucker for attachment alternately with the adherent foot, so that the animal loops its way along. In two families motile articulated rods occur; in Triarthridae they probably simply expand the dimensions of the body in adaptation to life at the surface; or as a protection against being swallowed by their smaller foes. In Polyurthra and Pteroessa they are numerous, pinnated (feathered), and are doubtless used for active swimming by jerks; they can be moved up or down by special muscles attached to their bases, which project into the body. In Pedalion (fig. 5), a remarkable form discovered by Dr C. J. Hudson in 1871 and found in numbers several times since, these
