The pneumato phore arises from the ectoderm as a pit or invagination, part of which forms a gas-secreting gland, while the rest gives rise to an air-sack lined by a chitinous cuticle.
The orifice of invagination forms a pore which may be closed up or may form a protruding duct or funnel.
Hence the cavity of the air-sack is equivalent to a sub-umbral cavity in which no manubrium is formed, and the pore or orifice of invagination would represent the margin of the umbrella.
The planula becomes elongated and broader towards one pole, at which a pit or invagination of the ectoderm arises.
The planula develops, on the whole, in a similar manner, but the ectodermal invagination arises, not at the pole of the planula, but on the side of its broader portion, and gives rise, not to a pneumatophore, but to a nectocalyx, the primary swimming bell or protocodon (" Fallschirm ") which is later thrown off and replaced by secondary swimming bells, metacodons, budded from the coenosarc.
There is but very little food-material in the egg of this Pectinibranch, and consequently the diblastula forms by invagination; the blastopore or orifice of invagination coincides with the anus, and never closes entirely.
There is, strictly speaking, no invagination (emboly), but an overgrowth (epiboly) of the smaller cells to enclose the larger.
The preconchylian invagination or shell-gland is formed in the embryo behind the velum, on the surface opposite the blastopore.
B, Trochosphere of an Opisthobranch (Pleurobranchidium) showing - shgr, the shell-gland or primitive shell-sac; v, the cilia of the velum; ph, the commencing stomodaeum or oral invagination; ot, the left otocyst; pg, red-coloured pigment spot.
Pedicle of invagination; its f, The foot.
The other extreme end closes, but the invaginated endoderm cells remain in continuity with this extremity of the blastopore, and form the " rectal peduncle " or " pedicle of invagination " of Lankester, although the endoderm cells retain no contact with the middle region of the now closed-up blastopore.
The ciliated band of the left side of the velar area is indicated by a line extending from v to v; the foot f is seen between the pharynx ph and the pedicle of invagination pi.
This bilobed sac becomes entirely the liver in the adult; the intestine and stomach are formed from the pedicle of invagination, whilst the pharynx, oesophagus and crop form from the stomodaeal invagination ph.
In some cases the inner layer is formed not by invagination but by proliferation or by de lamination.
- In the adult insect the head is insignificant in invagination, and are from their origin distinct from the mesoderm.
The hypoblast in some forms originates by invagination, in others by delamination.
The proboscis is an invagination from the epiblast; the proboscidian sheath appears in the mesoblast, but is perhaps originally derived from the hypoblast.
Monomeniscous eyes with a double (really triple) cell-layer formed by invagination, as in the Eu-arachnida, are present.
This is a process of gastrulation by invagination which is found in all animals above the Coelenterata, but which is very rare in the Cnidaria, and is known only in the Scyphomedusae amongst the Hydrozoa.
Gastrulation takes place by epiboly, and the stomodaeum (oral invagination - mastax pharynx) takes place in two stages of the region of the closed blastopore.
Un like the molluscs and annelids, however, the cloacal invagination lies outside this region, and the foot is formed by an elongation of the end of the body between the two apertures.
The organs are developed as invaginations of the epidermis of the foot, and in the majority of the Protobranchia the orifice of invagination remains open throughout life; this is also the case in Mytilus including the common mussel.
The shell-gland, sk, is later stage, in which a flattened out, and a delicate second invagination has beshell, s, appears on its sur gun - namely, that of the face.
25), no part of the blastopore persists either as mouth or as anus, but the aperture closes - the pedicle of invagination, or narrow neck of the invaginated arch-enteron, becoming the intestine.
The little mass of hypoblast or enteric cell-mass now enlarges, but remains connected with the cicatrix of the blastopore or orifice of invagination by a stalk, the rectal peduncle.
The enteron itself becomes bilobed and is joined by a new invagination, that of the mouth and stomodaeum.
In common with all other Coelomata, the Mollusca have the mouth and first part of the alimentary canal which leads into the met-enteron formed by a special invagination of the outer layer of the primitive body-wall, not to be confounded with that which often, but not always, accompanies the antecedent formation of the archenteron; this invagination is termed the stomodaeum.
Similarly an anal aperture is formed in connexion with a special invagination which meets the hinder part of the met-enteron, and is termed the proctodaeum.
By invagination one portion of this sphere becomes tucked into the other - as in the preparation of a woven night-cap for the head.
The orifice of invagination (blastopore) narrows, and we now have a two-cell-layered sac - the gastrula.
At the same time the stomodaeum, or oral invagination, forms around; the anterior remnant of the blastopore, and the proctodaeum, or anal invagination, forms around the posterior remnant of the blastopore.
Lastula, to show the orifice of invagination of the endo pallialis.
Pi, The pedicle of invagination, the future rectum.
Segmentation is total and at first regular, and is followed by invagination, the blastopore passing to the position of the future mouth.
An ectodemic invagination forms a large mucous gland on the foot, which is more or less atrophied in adult life.
2, 1) is a hollow organ enclosing an outgrowth of the body-cavity which is filled with fluid, and with its flexible under-surface capable of invagination or protrusion.
The embryo now consists of two layers of cells, epiblast and hypoblast, surrounding a cavity, the archenteron, which opens to the exterior by the orifice of invagination or blastopore.
After invagination is completed, the embryo begins to elongate, the blastopore becomes narrower, and the dorsal wall of the gastrula loses its convexity, and becomes flattened to form the dorsal plate, the outer layer of which is the primordium of the neurochord and the inner layer the primordium of the notochord.
In the Coelentera the ectoderm and endoderm are set apart from one another at a very early period in the life-history; generally either by delamination or invagination, processes described in the article Embryology.
To, A) by invagination (see article Hydrozoa).
Io, B) in the gastrula stage, and the orifice of invagination or blastopore, which persists, is situated at the hinder pole.
In the majority of the Decapoda there is a saccular invagination of the integument in the basal segment of the antennular peduncle having on its inner surface " auditory " setae of the type just described.
The hypoblast is formed either by a definite invagination or by the immigration of isolated cells, known as vitellophags, which wander through the yolk and later become associated into a definite mesenteron, or by some combination of these two methods.
Regarding the Echinoderms as a whole in the light of the foregoing account, we may give the following analytic summary of the characters that distinguish them from other coelomate animals: They live in salt or brackish water; a primitive bilateral symmetry is still manifest in the right and left divisions of the coelom; the middle coelomic cavities are primitively transformed into two hydrocoels communicating with the exterior indirectly through a duct or ducts of the anterior coelom; stereom, composed of crystalline carbonate of lime, is, with few exceptions, deposited by special amoebocytes in the meshes of a mesodermal stroma, chiefly in the integument; reproductive cells are derived from the endothelium, apparently of the anterior coelom; total segmentation of the ovum produces a coeloblastula and gastrula by invagination; mesenchyme is formed in the segmentation cavity by migration of cells, chiefly from the hypoblast.