290 



NATURAL HISTORY. 



PLUMULARIA HALECOIDES ( YOUNG). (After Hinclcs.) 



to the height of two inches. The calycles are narrow and cylindrical, and the aperture has three 

 denticles. The reproductive capsules are large and very deeply grooved or cut. It is a North Sea form. 



The genus Sertularia has the hydrothecse in two series 

 along the stem, and they are opposite or alternate, without 

 an external operculum. The gonothecse are large, scattered, 

 and have a simple orifice. 



The Sea Oak Coralline* is a common example, and 

 covers the fronds and stems of the larger seaweeds on the 

 British coasts. It is of a dusky horn-colour. 



FAMILY PLUMULARIID^E. 



The hydrothecse are sessile, and on one side nemato- 

 phores exist, and the polypites have a single wreath of 

 filiform tentacles round a central proboscis. The reproduc- 

 tive zooids are always fixed. 



Plumularia pinnata attains the height of four or 

 seven inches, and its stems are tall and whitish, and 

 jointed irregularly. The nematophores are sessile and 

 minute, one being below each calycle. The gonothecse form 

 a double row along the main stem, and have a number of 

 spinous projections at the tip. It is a common species on 

 shells, from low water to greater depths. The nematophores 

 consist of an extension of the body, which may be tubular, 

 or cup-shaped, or conical, open at the upper extremity, and 

 enclosing a granular mass, in which large thread cells may 

 be embedded. Some are simple, and the chitinous cup con- 

 sists of a single chamber, much adherent by its side, and when they are compound there is a 

 tubular portion below, expanding into a hemispherical cup. Some are pedunculate and others sessile. 

 They occur on various parts of the colony, and are usually 

 present in numbers about the hydrothecse: and in the genus 

 Aglaophenia every tooth on the crested ribs of the case or 

 gonangium which protects the gonophore, is formed by one of 

 them. The soft granular mass filling the nematophore has the 

 power of emitting and retracting very extensile and changeable 

 processes. 



In the genus Ophiodes remarkable thread-like organs are 

 found in great numbers on the creeping stolon, and one is 

 stationed close to the polypite. Each resembles a delicate ten- 

 tacle, has its narrow base surrounded by a cup-shaped prolonga- 

 tion of the outside tissue of the colony, and is terminated by 

 a knob with thread cells. 



In the family Campanularia the hydrothecfe have a ringed 

 peduncle, and the crown of tentacles is below their projecting 

 mouth-trunk. The gonophores are sessile or may become 

 transformed into medusae, some of which are flat and others 

 bell-shaped. 



The family Thaumantidse has medusae in the shape of long 

 bells with a short peduncle, the mouth being lobed. There are 

 two long and two rudimentary tentacles in the genus Lafoea, 

 and four radial canals, which contain the reproductive organs 

 in the form of ribbon-shaped masses. In the genus Melicertum the bell is shorter and broader 

 at the margin, and has a crowd of very slender irregular tentacles. When the eggs are hatched, 



* Sertularia, puniila. 



MEDUSA OF A HYDitoiD. (After Hinclcs.) 



