MICROSCOPICAL STUDIES. 95 



'cells derived from the downgrowth of two lines of cells from the 

 lining of the oesophagus (ectoderm cells*), the thickened edges of the 

 remaining mesenteries are less ciliate and their cells are endodermal 

 in origin. By the combined action of the cilia of the two dorsal 

 mesenterial edges, an ascending current of water is kept in motion. 



The gastric cavities of the zooids are sometimes very long, 

 extending downwards along the axis of the colony for several inches. 

 Those, of adjoining zooids are put into communication by a network 

 of fine ramifying tubes, buds from whose walls produce the fresh 

 zooids of the commonwealth. 



The group Alc} 7 onaria is of some importance geologically. Thus 

 Syringopora of Palaeozoic times was probably allied to the Organ- 

 Pipe Coral (Tubipora). The common fossil Favosites was also an 

 Alcyonarian, while Coralliwm has been found in the Jurassic rocks. 



Study XL — The Life-Cycle of Obelia geniculata. 



A few weeks since, a friend in the North of England was good 

 enough to send me some living specimens of a Zoophyte not found in 

 Jersey waters. Confined in a comparatively small jar, the somewhat 

 unnatural conditions had apparently hastened the birth of the 

 developing reproductive buds, and these were swimming freely in 

 the water as so many rythmically pulsating salver-shaped jelly fishes 

 of the most wondrous transparency and delicacy of form. A circlet 

 of regular and rather short thread-like arms stood out from the 

 margin, while upon the disc could just be made out, with the aid of 

 a lens, two intersecting lines with a handle-like stalk pendant from 

 the point where these two lines crossed. And the tiniest pin's head 

 was not larger in size. Some were born before my eyes from ovate 

 sacs in the branch axils of the Hydroid parent. 



Very generally the Hydrozoa, to which ' the subject of our 

 sketch belongs, present well-marked instances of that many phased 

 strange phenomenon, known familiarly as Alternation of Generation. 

 At one period living attached to rocks, or weeds, or shells, under 

 the form of tiny anemone-like animals, they often form by a con- 

 tinued process of budding, a shrub-like colony of many individuals. 

 This is termed the Hydroid Stage, from which arises by budding 

 and fission, free-swimming individuals or MedllS28 — the true sexual 



* Ectoderm and endoderm are terms of extreme importance. The embryo very 

 early in its history consists of two layers of cells, an outer ectoderm and an inner 

 endoderm, between which develops quickly a third, the mesoderm. All succeeding 

 tissues owe origin to one or other of these, and roughly we may take it that 

 epidermal tissues have ectodermal origin ; the alimentary canal and its offshoots 

 being lined with endoderm, while muscle, connective tissue, &c., are of mesodermal 

 origin. * * 



