236 Proceedings of the 



freshly aerated water. Its structure has a finely dotted or 

 crapy appearance. The dots show a tendency to arrange 

 themselves in transverse lines during the contraction of the 

 membrane, but no true muscular fibres exist. Numerous 

 transparent corpuscles may also be observed scattered with 

 some regularity in the tissue of the sub-umbrella, which, from 

 observations in other classes of zoophytes, I am led to consider 

 as the rudiments of a ganglionic nervous element. Similar 

 bodies, of a fusiform shape, may also be detected along the 

 sides and inner aspect of the lateral canals. The sub-um- 

 brella passes across the mouth of the bell to form the veil, which 

 is perforated in the centre by a wide circular opening. This 

 apparatus, by contracting the jet of fluid ejected from the 

 bell, increases its propulsive force ; at the same time, by its 

 projection outwards at each stroke of the sub-umbrella, it ren- 

 ders the stem, also, of our little vessel conical, and facilitates 

 its passage through the water. 



18. The alimentary polyp or peduncle (the digestive organ 

 of the Acaleph) rises from, and communicates with, the con- 

 fluence of the lateral tubes at the summit of the umbrella. In 

 shape it is short, quadrangular, and without tentacles, differ- 

 ing in this respect from the Acalephs of other species of Eu- 

 dendrium described by Dalyell and Van Beneden. Its struc- 

 ture consists of a layer of highly vacuolated endoderm, covered 

 by a delicate ectodermic investment, containing a fringe of 

 thread-cells round the mouth. 



19. The tentacular polyps (or prehensile organs) are placed 

 one at each confluence of the circular with the lateral canals. 

 They consist of two parts, — the hollow bulb and the tentacle. 

 In two of these polyps the tentacle is very short, while in the 

 other two, opposite to each other, it is excessively developed, 

 and, when not in use, coiled in a tight spiral. The ectoderm 

 or outer edge of the tentacle, or that which, when coiled, forms 

 the periphery of the spiral, is loaded with small thread-cells, 

 while its inner edge is destitute of those bodies. 



No eye-specks or otolithes can be detected in this Acaleph 

 with the microscope, but when a taper is brought close to the 

 side of the vessel in which it is swimming, a brilliant star of 

 light appears at each of the bulbs of the tentacular polyps, 



