In the Thallophytes the cytoplasm may be segmented by constriction, due to the in-growth of a new cell wall from the old one, as in Spirogyra and Cladophora, or by the formation of cleavage furrows in which the new cell-wall is secreted, as occurs in the formation of the spores in many Algae and Fungi.
The head - carrying feelers, mandibles and two pairs of maxillae - is succeeded by the three thoracic segments, each bearing a pair of strong five-segmented legs, whose feet, like those of the adult, carry two claws.
Extremes we find various transitional forms: an active larva, as described above, but with four-segmented, single-clawed legs, as among the rove-beetles and their allies; the body well armoured, but slender and worm-like, with very short legs as in wireworms and mealworms (figs.
Sharp (1899), who divides the order into six great series of families: Lamellicornia (including the chafers and stag-beetles and their allies with five-segmented feet and plate-like terminal segments to the feelers); Adephaga (carnivorous, terrestrial and aquatic beetles, all with five foot-segments); Polymorpha (including a heterogeneous assembly of families that cannot be fitted into any of the other groups); Heteromera (beetles with the fore and intermediate feet five-segmented, and the hind-feet four-segmented); Phytophaga (including the leaf-beetles, and longhorns, distinguished by the apparently four-segmented feet), and Rhynchophora (the weevils and their allies, with head prolonged into a snout, and feet with four segments).
Ganglbauer (1892) divides the whole order into two sub-orders only, the Caraboidea (the Adephaga of Sharp and the older writers) and the Cantharidoidea (including all other beetles), since the larvae of Caraboidea have five-segmented, two-clawed legs, while those of all other beetles have legs with four segments and a single claw.
- This tribe includes beetles of carnivorous habit with five segments on every foot, simple thread-like feelers with none of the segments enlarged to form club or pectination, and the outer lobs (galea) of the first maxilla usually two-segmented and palpiform (fig.
2 c) are active, with well-chitinized cuticle, often with elongate tail-feelers (cerci), and with five-segmented legs, the foot-segment carrying two claws.
The curious little Pselaphidae, with three-segmented tarsi, elongate palpi, and shortened abdomen; the latter are usually found in ants' nests, where they are tended by the ants, which take a sweet fluid secreted among little tufts of hair on the beetles' bodies; these beetles, which are carried about by the ants, sometimes devour their larvae.
The female is a segmented, wormlike creature, spending her whole life within the body of the bee, wasp or bug on which she is parasitic. One end of her body protrudes from between two of the abdominal segments of the host; it has been a subject of dispute whether this protruded end is the head or the tail, but there can be little doubt that it is the latter.
Linnaeus in his Systema naturae (1735) grouped under the class Insecta all segmented animals with firm exoskeleton and jointed limbs - that is to say, the insects, centipedes, millipedes, crustaceans, spiders, scorpions and their allies.
They never bear segmented limbs (palps) and only exceptionally (as in the chafers) is the skeleton composed of more than one sclerite.
In their typical state of development, the first maxillae offer a striking contrast to the mandibles, being composed of a two-segmented basal piece (cardo and stipes, fig.
Carriere's researches (1897) on the make its appearance simultaneously throughout the whole length of embryology of the mason bee (Chalicodoma) agree entirely with the the plate; the anterior parts are segmented before the posterior.
- The nervous system is ectodermal in origin, and is developed and segmented to a large extent in connexion with the outer part of the body, so that it affords important evidence as to the segmentation thereof.
- Morphology of an Insect: the embryo of Gryllotalpa, somewhat diagrammatic. The longitudinal segmented band along the middle line represents the early segmentation of the nervous system and the subsequent median field of each sternite; the lateral transverse unshaded bands are the lateral fields of each segment; the shaded areas indicate the more internally placed mesoderm layer.
I t may, however, be pointed out that Brachiopods seem to belong to that class of animal which commences life as a larva with three segments, and that tri-segmented larvae have been found now in several of the larger groups.
Appendages of 1st pair.tri-segmented, chelate; of 2nd pair chelate, with their basal segments subserving mastication; of 3rd, 4th, 5th and 6th pairs similar in form and function, except that in recent and Carboniferous forms the basal segments of the 3rd and 4th are provided with sterno-coxal (maxillary) lobes, those of the 4th pair meeting in the middle line and underlying the mouth.
Paf, Post-anal segmented filament corresponding to the post-anal spine of Limulus.
Opisthosoma when segmented showing the same number of somites as in the Pedipalpi; usually unsegmented, the prae-genital somite constricted to form the waist; the appendages of its 3rd and 4th somites retained as spinning mammillae.
- Opisthosoma dis tinctly segmented, furnished with tergal plates, as in the Ambly pygi; the ventral surface of the 1st and 2nd somites with large sternal plates, covering the genital aperture and the two pairs of FIG.
Appendages of 1st pair, bi-segmented, completely chelate.
Appendages of 1st pair large, three segmented and completely chelate; of 2nd pair either simple and pediform, or prehensile and subchelate; of remaining four pairs, similar in form, ambulatory in function; the basal segment of the 2nd, 3rd and sometimes of the 4th pairs of appendages furnished with sterno-coxal (maxillary) lobe.
E, Lateral view of the whole body and two 1st appendages, showing the fusion of the dorsal elements of the prosoma into a single plate, and of those of the opisthosoma into an imperfectly segmented plate continuous with that of the prosoma.
- Degenerate atracheate gall-mites in which the body is produced posteriorly and annulated, as in Demodex, but in which the appendages of the 3rd and 4th pairs are long and normally segmented and those of the 5th and 6th pairs entirely absent.
TAPEWORMS. The Cestodes or Tapeworms form a class of purely endoparasitic Platyelmia, characterized by their elongate shape, segmented bodies, and the absence of a digestive system.
In contrast to these segmented or " merozoic " Cestodes, a few primitive forms have preserved a unisegmental' character and form the Monozoa or Cestodaria.
The nervous system is, however, not segmented, and the excretory system is continuous throughout the worm.
We therefore regard the body of a Cestode as a single organism within which the gonads have become segmented, and the segmentation of the body as a secondary phenomenon associated with diffuse osmotic feeding in the narrow intestinal canal.
Schistocephalus becomes fully segmented in Gasterosteus and mature in aquatic birds (ducks, &c.).
Triaenophorus, indistinctly segmented, occurs in the pike.
By a series of changes similar to those by which the primary larva arose from a segmented egg, so do these secondary larvae or "rediae" arise from the germ-cells or germ-balls within the sporocyst.
The nervous system consists of a ganglion or brain, which lies dorsally about the level of the junction of the pharynx and the stomach, a nerve ring and a segmented neutral cord.
This division resembles the Cynipoidea in the position of the ovipositor, and in the two segmented trochanters.
In the curious little Tingidae, whose integuments exhibit a pattern of network-like ridges, the feet are two-segmented and the scutellum is hidden by the pronotum.
The Aradidae have two segmented feet, and a large visible scutellum.
The Cimicidae have the feet three-segmented and the forewings greatly reduced; most of the species are parasites on birds and bats, but one - Cimex lectularius (figs.
The Naucoridae and Belostomatidae are flattened insects, with four-segmented feelers and fore-legs inserted at the front of the prosternum.
Have the head with two ocelli and three-segmented feelers; frequently as in the tropical " lantern-flies " (q.v.) the head is prolonged into a conspicuous bladder, or trunk-like process.
Annelids are segmented worms, and differ from the Arthropoda, which they closely resemble in many respects, by the possession of a portion of the coelom traversed by the alimentary canal.
They possess a modified anterior end, frequently with special sense organs, forming a head, a segmented nervous system, consisting of a pair of anterior, dorsally-placed ganglia, a ring surrounding the A ? ??° D B L/ alimentary canal, and a double ventral ganglionated chain, a definite vascular system, an excretory system consisting of nephridia, and paired generative organs formed from the coelomic epithelium.
The Copepoda have normally a segmented body, not enclosed in a bivalved shell-covering, the segments not exceeding eleven, the limbs not branchial.
- The body, seldom in any way segmented,, is wholly encased in a bivalved shell, the caudal part strongly inflexed, and almost always ending in a furca.
(e) Cytherellidae, which, unlike the Ostracoda in general, have the hinder part of the body segmented, at least ten segments being distinguishable in the female.
The next, the copepodid or cyclopid, stage is characterized by a cylindrical segmented body, with foreand hind-body distinct, and by having at most six cephalic limbs and two pairs of swimming feet.
In the Limnephilidae the maxillary palp is three-segmented in the male, the larvae are variable in habit, many forming cases of snail-shells.
The males of the Sericostomatidae have two or three segmented palps; their larvae inhabit running water and make cases of grains of sand, or of small stones.
This fact gives rise to the question whether the Mollusca are to be regarded as primitively segmented animals or not.
In animals which exhibit typical segmentation or metamerism, such as segmented worms (Chaetopoda), each segment or metamere possesses its own coelomic cavity, a pair of coelomic ducts, and a pair of nephridia.
The Locustidae (see Grasshopper, Katydid) have the feelers and often also the ovipositor very elongate; the foot is four-segmented; the ears are placed at the base of the foreshin and the stridulation is due to the friction of a transverse " file " beneath the base of the left forewing over a sharp ridge on the upper aspect of the right.
The cellular process from the axis which develops into a leaf is simple and undivided; it rarely remains so, but in progress of growth becomes segmented in various ways, either longitudinally or laterally, or in both ways.