The medial portion forms radiating tracts of fibres, the so-called " bell-muscles " running underneath, and parallel to, the radial canals; when greatly developed, as in Tiaridae, they form ridges, so-called mesenteries, projecting into the sub-umbral cavity.
The body is composed of a large number of segments; the prostomium bears a pair of tentacles; the nervous system consists of a brain and longitudinal ventral nerve cords closely connected with the epidermis (without distinct ganglia), widely separated in Saccocirrus, closely approximated in Protodrilus, fused together in Polygordius; the coelom is well developed, the septa are distinct, and the dorsal and ventral longitudinal mesenteries are complete; the nephridia are simple, and open into the coelom.
The Hydromedusae are distinguished from the Scyphozoa chiefly by negative characters; they have no stomodaeum, that is, no ingrowth of ectoderm at the mouth to form an oesophagus; they have no mesenteries (radiating partitions) which incompletely subdivide the coelenteron; and they have no concentration of digestive cells into special organs.
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.
The sexual cells are borne on the mesenteries in positions irrespective of obvious developmental radii.
At the parting of the ways which led, on the one hand, to modern Scyphomedusae, on the other to Anthozoa (III.), it is probable that the common ancestor was marked by incipient mesenteries and by the limitation of the sexual cells to endoderm.
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.
It appears, however, that the first of these characters is non-existent, and that the so-called mesenteries are simply the concrescence-areas found in all medusae.
These radial folds are known as mesenteries, and their position and relations may be understood by reference to figs.
A certain number of mesenteries, known as complete mesenteries, are attached by the upper parts of their internal margins to the stomodaeum, but below this level their edges hang in the coelenteron.
Other mesenteries, called incomplete, are not attached to the stomodaeum, and their internal margins are free from the peristome to the basal disk.
4 organs or gonads are borne on the mesenteries, the germinal cells being derived from the inner layer or endoderm.
There are always eight tentacles, which are hollow and fringed on their sides, with hollow projections or pinnae; and always eight mesenteries, all of which are complete, i.e.
The mesenteries are provided with well-developed longitudinal retractor muscles, supported on longitudinal folds or plaits of the mesogloea, so that in cross-section they have a branched appearance.
In these the tentacles are stunted or suppressed and the mesenteries are ill-developed, but the sulcus is unusually large and has long cilia.
Mm, Mesenteries; budding from a single mb muscle banners; sc, sulcus; st, parent zooid.
St, Stomodaeum in the centre of the transparent body; m, mesenteries; asm, asulcar mesenteries; B, spicules, enlarged.
There are from sixteen to thirty-two simple tentacles, but only eight mesenteries, all of which are complete.
The arrangement of the muscle-banners on the mesenteries is characteristic. On six of the mesenteries the muscle-banners have the same position as in the Alcyonaria, namely, on the sulcar faces; but in the two remaining mesenteries, namely, those which are attached on either side of the sulcus, the muscle-banners are on the opposite or sulcular faces.
It is not known whether all the eight mesenteries of Edwardsia are developed simultaneously or not, but in the youngest FIG.
D, d, Directive mesenteries; st, stomodaeum.
Form which has been studied all the eight mesenteries were present, but only two of them, namely the sulco-laterals, bore mesenterial filaments, and so it is presumed that they are the first pair to be developed.
In the common sea-anemone, Actinia equina (which has already been quoted as a type of Anthozoan structure), the mesenteries are numerous and are arranged in cycles.
The mesenteries of the first cycle are complete (i.e.
In the four couples of mesenteries which are attached to the sides of the elongated stomodaeum the muscle-banners of each couple are turned towards one another, but in the sulcar and sulcular couples, known as the directive: FIG.
The space enclosed between two mesenteries of the same couple is called an entocoele; the space enclosed between two mesenteries of adjacent couples is called an exocoele.
The second cycle of mesenteries consists of six couples, each formed in an exocoele of the primary cycle, and in each couple the muscle-banners are vis-a-vis.
Next, a pair of mesenteries, marked II,II in the diagram, is developed in the sulcular chamber, its musclebanners facing the same way as those of I, I.
There are now eight mesenteries present, having exactly the same arrangement as in Edwardsia.
A pause in the development follows, during which no new mesenteries are formed, and then the six-rayed symmetry characteristic of a normal Actinian zooid is completed by the formation of the mesenteries V, V in the lateral chambers, and VI, VI in the sulcolateral chambers, their muscle-banners being so disposed that they form couples respectively with II, II and I, I.
The mesenteries second in order of formation form the sulcular directives, those fourth in order of formation form with the fifth the sulculo-lateral couples of the adult.
Externally they resemble ordinary sea-anemones, but there is only one ciliated groove, the sulcus, in the stomodaeum, and the mesenteries are arranged on a peculiar pattern.
The first twelve mesenteries are disposed in couples, and do not differ from those of Actinia except in size.
The subsequent development is peculiar to the group. New mesenteries are formed only in the sulco-lateral exocoeles.
The mesenteries are numerous, and the longitudinal muscles, though distinguishable, are so feebly developed that there are no musclebanners.
In this larva four pairs of mesenteries having the typical Edwardsian arrangement are developed, but the fifth and sixth pairs, instead of forming couples with the first and second, arise in the sulcar chamber, the fifth pair inside the fourth, and the sixth pair inside the fifth.
In the Cerianthidea, as in the Zoanthidea, much as the adult arrangement of mesenteries differs from that of Actinia, the derivation from an Edwardsia stock is obvious.
There are ten mesenteries in which the musculature is so little developed as to be almost indistinguishable.
The sulcar and sulcular pairs of mesenteries are FIG.
Short, the sulco-lateral and sulculo-lateral pairs are a little longer, but the two transverse are very large and are the only mesenteries which bear gonads.
As the development of the Antipathidea is unknown, it is impossible to say what is the sequence of the mesenterial development, but in Leiopathes glaberrima, a genus with twelve mesenteries, there are distinct indications of an Edwardsia stage.
The septa in recent corals always bear a definite relation to the mesenteries, being found either in every entocoele or in every entocoele and exocoele.