Highly specialized air-sacs are characteristic of all birds.
To this category belong certain sacs and pouches.
Air-sacs, by which the breathing capacity is much increased.
In some cases special secreting tissues, resin ducts, oil glands, laticiferous tissue, crystal sacs, &c., may be developed among the ordinary secondary vascular elements.
The formation of the conducting tubes or secretory sacs which occur in all parts of the higher plants is due either to the elongation of single cells or to the fusion of cells together in rows by the absorption of the cell-walls separating them.
The ventral portion of the neck is formed by the strong crista inferior, on the median side of which is the deep fosses subtrochanterica by which air sacs enter the humerus.
Exchange of gas through the walls of the air-sacs, almost devoid of blood-vessels, can at best be much restricted.
Special sacs developed from the intersegmental septa lodge the developing ova and sperm.
It is indeed only among the Eudrilidae that the enclosure of the ovaries in septal sacs is at all general.
There are, however, in the Eudrilidae, as already mentioned, sacs envolving the ovaries which bore their own way to the exterior, and thus may be termed coelomoducts.
These sacs are dealt with later under the description of the spermathecae, which function they appear to perform.
They are essentially spherical, pear-shaped or oval sacs opening on to the exterior but closed at the coelomic end.
12 and 13 are shown the spermathecae of the genera Hyperiodrilus and Heliodrilus, which are simple sacs ending blindly as in other earthworms, but of which there is only one median opening in the thirteenth segment or in the eleventh.
Sperm sacs generally occupying a good many segments and with simple interior undivided by a network of trabeculae.
Egg sacs as large or nearly so as sperm sacs.
Sperm ducts and atria as in Limicolae; egg sacs large; body wall thick; vascular system and nephridia as in Terricolae.
Sperm sacs generally limited to one or two segments with interior subdivided by trabeculae.
The two long ovarian sacs communicate with each other by a transverse bridge before uniting to form the terminal canal.
The large sacs which have been termed vagina are suggestive of the large coelomic spermathecae in Eudrilids, a comparison which needs, however, embryological data, not at present forthcoming, for its justification.
The walls of the renal sacs are deeply plaited and thrown into ridges.
The oesophagus leads into a remarkable stomach, plaited like the manyplies of a sheep, and after this the intestine takes a very large number of turns embedded in the yellow liver, until at last it passes between the two renal sacs to the anal papilla.
Some amongst them (Tergipes, Eolis) are also remarkable for possessing peculiarly modified cells placed in sacs (cnidosacs) at the apices of these same papillae, which resemble the " thread-cells " of the Coelentera.
It seems probable that it is identical with one of the open sacs in which each shell-plate of a Chiton is formed, and the series of plate-like imbrications which are placed behind the single shell-sac on the dorsum of the curious slug, Plectrophorus, suggest the possibility of the formation of a series of shellsacs on the back of that animal similar to those which we find in Chiton.
The significance of two or more (in Drepanophorus very numerous) small sacs containing so-called " reserve " stylets resembling in shape that of the central dart is insufficiently known.
A series 750.) I, The longitudinal of sacs lined with an epithelium, the proexcretory canal; 2, one liferation of which gives rise to the ova of the tags containing the or spermatozoa, alternate between the flame-cells.
It has been pointed out that the cavity of the sacs corresponds in many particulars with the coelom of higher animals, and in Lebidinsky's observations on the development there is some support to the view that a coelom exists.
Both male and female gonads consist of more or less lobulated hollow sacs connected with the epidermis by short ducts.
Latex, though chiefly secreted in vessels or small sacs which reside in the cortical tissue between the outer bark and the wood is also found in the leaves and sometimes in the roots or bulbs.
It consists of a definite contractile sac or sacs lying on the dorsal side of the alimentary canal near the oesophagus, and in preparations of Terebratulina made by quickly removing the viscera and examining them in sea-water under a microscope, he was able to count the pulsations, which followed one another at intervals of 30-40 seconds.
In specialized forms these pulmonary sacs are wholly or partly replaced by tracheal tubes.
The appearance of tracheae in place of lung-sacs cannot be regarded as a starting-point for a new line of descent comprising all the tracheate forms; From Lankester, Journ.
The injections are made by means of a hypodermic syringe into the subcutaneous tissue, into a vein, into one of the serous sacs, or more rarely into some special part of the body.
1, ii.-ix.), accompanied by paired eversible sacs, probably respiratory in function - on eight (or fewer) other abdominal segments.
The first of these usually carries a ventral tube, furnished with paired eversible sacs which assist the insects in walking on smooth surfaces, and perhaps serve also as organs for breathing.
The eversible sacs on the abdominal segments are shown, some protruded and some retracted.
The ovaries are epithelial sacs which open into the uteri.
(For further details on the form and arrangement of the flower and its parts, see Flower.) Each stamen generally bears four pollen-sacs (microsporangia) which are associated to form the anther, and carried up on a stalk or filament.
If in its extension to contain the new formations within it the embryo-sac remains narrow, endosperm formation proceeds upon the lines of a cell-division, but in wide embryo-sacs the endosperm is first of all formed as a layer of naked cells around the wall of the sac, and only gradually acquires a pluricellular character, forming a tissue filling the sac. The function of the endosperm is primarily that of nourishing the embryo, and its basal position in the embryo-sac places it favourably for the absorption of food material entering the ovule.
In Dinophilus there is also only a single pair of genital ducts behind; and in the male there are sperm-sacs and a median penis.
In Utricularia bladder-like sacs are formed by a modification of leaflets on the submerged leaves.
The reproductive spores are borne in sacs (asci) which form a dense layer on the surface, appearing like a bloom in July; they are scattered by the wind and propagate the disease.
The silk glands or vessels consist of two long thick-walled sacs running along the sides of the body, which open by a common orifice - the spinneret or seripositor - on the under lip of the larva.