(I) The polyp (hydropolyp) is of simple structure, typically much longer than broad, without ectodermal oesophagus or mesenteries, such as are seen in the anthopolyp (see article Anthozoa); the mouth is usually raised above the peristome on a short conical elevation or hypostome; the ectoderm is without cilia.
In the hydropolyp the ectodermal muscle-fibres are always directed longitudinally.
A large nematocyst, with everted thread, is seen in the right-hand ectodermal cell.
In contrast with the polyp, the longitudinal muscle-system is entirely ectodermal, there being no endodermal muscles in craspedote medusae.
Further, two distinct types of otocyst can be recognized in the Hydro medusae; that of the Leptolinae, in which the entire organ is ectodermal, concrement-cells and all, and the organ is not a tentaculocyst; and that of the Trachylinae, in which the organ is a tentaculocyst, and the concrement-cells are endodermal, derived from the endoderm of the modified tentacle, while the rest of the organ is ectodermal.
Covered with sensory ectodermal epithelium an d containing an After 0.
Oc, Pigmented ectodermal cells; 1, lens.
The endodermal spadix (sp) of the sporosac represents the endoderm of the manubrium; the ectodermal lining of the sporosac (ex.) represents the ex-umbral ectoderm of the medusa; and the intervening layers, together with the sub-umbral cavity, have disappeared.
There is, however, no evidence whatever for this, the gonads of Hydra being purely ectodermal structures, while all medusoid gono phores have an endodermal portion.
It is seen from the foregoing account of medusa - budding that the entocodon is a very important constituent of the bud, furnishing some of the most essential portions of the medusa; its cavity becomes the subumbral cavity, and its lining furnishes the ectodermal epithelium of the manubrium and of the sub-umbral cavity as far as the edge of the velum.
By a simple modification, the open pit becomes a solid ectodermal ingrowth, just as in Teleostean fishes the hollow medullary tube, or the auditory pit of other vertebrate embryos, is formed at first as a solid cord of cells, which acquires a cavity secondarily.
There is no free-swimming planula larva, but the stage corresponding to it is passed over in an enveloping cyst, which is secreted round the embryo by its own ectodermal layer, shortly after the germ-layer formation is complete, i.e.
The characteristic sense-organs are ectodermal otocysts, absent, however, in some genera, in which case cordyli may replace them.
It has been asserted that the tentaculocysts are entirely ectodermal and that either the family should be placed amongst the Leptomedusae, or should form, together with certain Leptomedusae, an entirely distinct order.
Thus the ectodermal rim runs round the edge of each lobe of the umbrella and then passes upwards towards the base of the tentacle from the re-entering angle between two adjacent lobes, to form with its fellow of the next lobe a tentacle-clasp or peronium, i.e.
The tentaculocysts are implanted round the margins of the lobes of the umbrella and may be supported by prolongations of the ectodermal rim termed otoporpae (Gehorspangen).
Haeckel regarded the whole structure as a glandular ectodermal pit formed on the exumbral surface of a medusa-person.
Woltereck , on the other hand, have shown that the ectodermal pit which gives rise to the pneumatophore represents an entocodon.
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.
In such forms as Velella, Porpita, &c., the ancestor was an eight-rayed medusa (Disconula) which acquired a pneumatophore as an ectodermal pit on the ex-umbrella, and in which the organs (manubrium, tentacles, &c.) became secondarily multiplied, just as they do in Gastroblasta as the result of incomplete fission.
Usually they open into a median vagina, formed by an ectodermal inpushing and lined with chitin.
- 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.
The median vagina, spermatheca and ejaculatory duct are, on the other hand, formed by ectodermal inpushings.
In the absence of the external ectodermal ducts usual in winged insects, these two groups resemble therefore the primitive Aptera.
Without ectodermal genital ducts.
So the Dermaptera, which retain distinct maxillulae and have no ectodermal genital ducts, have either specialized or aborted wings and a large number of Malpighian tubes.
The independent opening of the genital ducts and the absence of an ectodermal vagina and ejaculatory duct are remarkable archaic features of these insects, as has been pointed out by J.
There is a special marginal nerve running round the edge of the mantle, but the connexion of this with the rest of the nervous system is not clear; probably it is merely another concentration of the diffused sub-ectodermal nervous fibrils.
The ectodermal cells are large, ciliated, and amongst the ciliated cells glandular cells are scattered.
The chitinous chaetae have their origin in special ectodermal pits, at the base of which is one large cell which is thought to secrete the chaeta, as in Chaetopods.
These pits are not isolated, but are connected by an ectodermal ridge, which grows in at the margin of the mantle and forms a continuous band somewhat resembling the ectodermal primordium of vertebrate teeth.
Ing the nerves are for the most part at the base of the ectodermal cells.
In the anthopolyp the blastopore is carried inwards by an in-pushing of the body-wall of the region of the peristome, so that the adult mouth is an opening leading into a short ectodermal oesophagus or stomodaeum, at the bottom of which is the blastopore.
The polyp (hydropolyp) is of simple structure, and never has an ectodermal oesophagus or mesenteries.
When the gonads are endodermal, they are formed on the floor of the stomach; when ectodermal (G, see fig.
The Dermaptera and agree with the vast majority of winged insects - are the absence of distinct maxillulae and the presence of an unpaired ectodermal tube as the terminal region of the genital system in both sexes.
The skin consists of a layer of cuticle, easily stripped off, secreted by an ectodermal layer one cell thick.
Their ectodermal muscles are mainly longitudinal, their endodermal muscles are circularly arranged on the body-wall.
The Scyphozoa have the following features in common: - They typically exhibit an ectodermal stomodaeum; partitions or mesenteries project into their coelenteron from the body-wall, and on these are generally concentrated digestive cells (to form mesenterial filaments, phacellae or gastric filaments, &c.); the external musculature of the body-wall is circular (except in Cerianthus); the internal, longitudinal; and the sexual cells probably always arise in the endoderm.
In Hydromedusae, of Schafer.) two kinds of elements: (After (I) visual cells, sensory ectodermal cells, which may develop terminal visual cones; (2) pigment-cells, usually ectodermal, but in one known instance endodermal.
The simplest type of ocellus is exemplified by the exumbral ocellus of Aurelia, a simple patch of pigment-cells interspersed with visual cells, the whole on a level with the remaining ectodermal epithelium.
Ent, Canal of the enteric system oc, Ectodermal pigment (ocel- T, Tentaculocyst.
Highly developed eyes, with ectodermal pigment and lens, are found also on the rhopalia of Paraphyllina (Maas [81).
The subumbral ocellus of Aurelia is found to be of the inverted type, with the visual cones turned away from the light, as in Tiaropsis amongst Hydromedusae, and here also the pigment is furnished by the endoderm, forming a cup into which the ectodermal visual cells project (Schewiakoff ['31) In) the Stauromedusae tentaculocysts are either absent altogether, as in - St.
It has been sought to prove that the interior of the hypostome is lined by ectoderm, so as to form a stomodaeum or ectodermal oesophagus similar FIG.
Each taeniola bears a strongly developed longitudinal muscle-band, stated by Claus and Chun to be developed from the endoderm, like the retractor muscles of the anthopolyp, but by other investigators it is affirmed that each retractor muscle of the scyphistoma arises from the lining of a funnel-shaped ectodermal ingrowth (" Septaltrichter ") growing down from the peristome inside each taeniola, in a manner similar to the infundibular cavities of Lucernaria, which in their turn are homologous with the sub f genital cavities of Scypho l A .` medusae.
The diagnostic features of the class Scyphozoa thus constituted are supposed to be (I) an ectodermal oesophagus or stomodaeum, (2) a gastric cavity subdivided by mesenteries, (3) gonads formed in the endoderm.
As in all Arthropoda, it is composed of three divisions, a fore-gut or stomodaeum, ectodermal in origin and lined by an inturning of the chitinous cuticle, a mid-gut formed by endoderm and without a cuticular lining, and a hind-gut or proctodaeum, which, like the fore-gut, is ectodermal and is lined by cuticle.