REPORT OX THE HYDROIDA. 
XXV11 
they are bell-shaped, the walls of the bell or umbrella being mainly composed of a trans- 
parent substance of gelatinous consistence. From the summit of the bell-cavity hangs a 
tubular body of variable length, — the manubrium — whose distal or free extremity carries 
the mouth, and on whose cavity devolves the function of digestion. From the basis of 
the manubrium four (sometimes more) canals pass off and run in equally distant meridian 
lines in the walls of the umbrella towards its margin, where they open into a circular 
canal which runs quite round the margin of the umbrella. The opening ( codonostome ) 
of the bell-cavity is partially closed by a membranous diaphragm, the velum , which 
stretches across it from the margin, and is perforated in the centre by an aperture 
which allows the surrounding water to enter the bell, and through which it may be 
again expelled from within, while hollow contractile filaments or tentacles, which vary 
much in number in the different species, hang from the umbrella margin. 
The umbrella is eminently contractile, and acts by alternate rhythmical dilatations 
and contractions (diastole and systole) of its cavity as a powerful organ of locomotion by 
which the planoblast is propelled through the surrounding water. 
The external or convex, and the internal or concave, surfaces of the umbrella are 
clothed with a cellular epithelium — the representative of the ectoderm. Between these 
lies the thick elastic gelatinous substance traversed in meridional lines by the radiating 
canals, and in a circular direction by the marginal or circular canal. Both sets of canals 
have their walls lined with endoderm. 
The researches of the Hertwigs 1 have shown that a thin lamina of endoderm exists 
in the intervals of the radiating canals, and connects these canals laterally with one 
another. This “ endoderm lamella ” represents the obliterated portion of the lumen 
which existed between the two layers of the double walls of an endodermal cup which was 
present in the early stages of the developing bud, while the radiating canals represent 
those portions of the lumen which have continued pervious (see below, p. xxxi). 
The great contractility of the umbrella is due to a continuous muscular sheet which 
lies just under its concave or “ subumbrellar ” surface. The fibres of this muscular layer 
are transversely striated and seem to be derived from the epithelial cells. They take a 
circular course parallel to the umbrella margin. In some cases fascicles of meridionally 
disposed fibres are also present. A strong muscular layer also exists in the velum, where 
the internal surface of the stomach -walls. 3. The absence in the Craspedotae of true marginal lobes, into which the 
margin of the umbrella is divided in the Acraspedse. 4. The fact of the sexual receptacles (genitalia) of the Craspedotae 
discharging their contents externally into the surrounding water instead of internally into the cavity of the gastro- 
vascular system as in the Acraspedse. 5. The possession by the Craspedotae of a double marginal nerve-ring repre- 
senting a centralised nervous system, while in the Acraspedse where a nerve-ring has been demonstrated this is never 
double. 6. The fact of the polyp-form which shows itself in the course of the development of a Craspedote Medusa 
being a Hvdropolyp destitute of gastral longitudinal ridges (“ tseniola ” in the strict sense of the word), while in the 
Acraspedse it is a Scyphopolyp or polyp-form with gastral tseniola. 7. The fact of the Craspedote Medusa being formed 
by lateral budding from its Hydroid trophosome, while the Acraspedote Medusa is formed by terminal budding from 
its Scyphopolyp. 
1 0. und R. Hertwig, Das Nervensystem und die Sinnesorgane der Medusen, Leipzig, 1878. 
