THAUMANTIAS HEMISPHiERICA. 51 



in the Zetland Isles, to observe the mode of formation of their tentacula, a point of considerable 

 physiological importance. The results may be summed up as follows : — 



1st stage. The first indication of the tentacle is in the form of a minute lobe at the 

 margin of the body, in the immediate neighbourhood, and in contact with the motor band and 

 marginal vessel. 



2d. This lobe becomes a closed cell, which then rapidly enlarges in every direction, but 

 ehiefly superiorly and inferiorly. 



3d. It next presents a contraction, which is the commencement of its division into two 

 cells, the superior being largest. 



4th. The inferior cell grows rapidly, and divides itself into two, the lowermost of which 

 does the same, and this process is continued until the tentacle has elongated considerably in 

 one direction. In the mean time the superior vesicle of all enlarges, but does not divide. All 

 present the aspect of nucleated cells. 



5th. The partitions between the cells begin to be lined with minute, translucent granules, 

 which give the tentacula a moniliform aspect ; and from this time those organs become con- 

 tractile. 



6 th. The minute translucent granules (which are themselves cells of a secondary order) 

 multiply until they fill up the primary cells, and give the tentacula the aspect of homogeneous 

 bodies, composed of highly contractile granular tissue. 



7th. When the tentacle has far advanced towards its perfect state, the bulb-like vesicle 

 at its base also begins to be filled with granular tissue, many of the minute cells of which 

 become pigment-cells. 



8th. In the middle of this mass of cells, one cell appears larger than the last, and which, 

 increasing with great rapidity, divides in the end the mass of minute secondary cells into two 

 pad-like bodies, constituting together the conspicuous ocellus. 



9th. When this division has taken place, there appears in the central cell a new and free 

 mass of minute cells, in which are secreted crystals (of carbonate of lime). These are the 

 otolites. The tentacle is now complete. 



The Thaumantias hemisphaerica is an active little animal, gregarious. It is exceedingly 

 tenacious of life, as the tortures to which it was submitted by Dr. Macartney, quoted in my 

 introductory remarks, show. It is very abundant in the Zetland seas, and occurs also in the 

 east and west sides of the mainland of Scotland. It is also, according to Mr. W. Thompson, 

 found on the north coast of Ireland. " In October, 1838, 1 obtained one of these Medusae in 

 Belfast Bay, and a day or two afterwards many specimens were brought to me by Mr. 

 Hyndman from the same locaUty ; in size they rather exceed Miiller's, measuring five lines in 

 diameter in their most depressed state. Mr. R. Patterson informs me that he obtained the 

 M. hemisphcerica at Larne, in the summer of 1835," 



Plate VIII, f. 2, a, represents this species seen in profile, magnified ; 2, h, seen from 

 above, a very large specimen (too many tentacles and tubercles are introduced in the figure, 

 by mistake) ; 2, c, is one of the reproductive glands ; 2, d, those of the tentacles and their 

 bulbs. 



