516 THE MICROSCOPE. 



the tube from which it springs. This compartment is 

 formed, like those of the central disc, by a thick septum, 

 which shuts off the origin of the secondaiy branch from 

 the main cavity of the primary one." 



The larger, or polypiferoua cells, Mr. Busk proposes to 

 term cells, and the smaller tobacco-pipe-shaped organs 

 cups; the latter being usually above the former through- 

 out the polypidom, '^excepting immediately below each 

 fork, where the cup is invariably absent above one of the 

 cells of the pair from between which the fork springs. 

 The polype-cells are several times larger than the cups, 

 and their walls are much thinner ; in fact, sufficiently 

 transparent to allow of the contents of the cell being 

 pretty well seen, without any preparation, even during the 

 life of the animal. In shape they are inversely conical, 

 and the outer and upper angle is usually produced into a 

 prominent, sharp point. From the internal and upper 

 angle arises the tubular prolongation going to form the 

 next cell or cup, as the case may be, in succession. They 

 are entirely closed at the top, contrary to what is stated 

 by previous observers ; and, as has been shown, there is no 

 connexion whatever between the cell and the cup placed 

 immediately above and behind it. The aperture of the cell 

 is on the anterior face, and towards the upi)er margin ; it 

 is of a crescentic form, and placed obliquely, as it were, 

 across the upper and internal angle of the cell, with the 

 convexity of the curve directed upwards and inwards. The 

 lips of the aperture are strengthened by thin bands of 

 horny material ; and, under favourable circumstances, 

 indications of short muscular fibres, for the purpose of 

 opening or closing the aperture, may be seen." 



The shell, which Mr. Busk believes to be entire at the 

 bottom, though closed only by a very delicate membrane, 

 contains an ascidoid polype of the usual typical form of 

 that class. " It has ten tentacles, and no gizzard. Two 

 sets of muscular fibres at least may be distinguished as 

 appertaining to the polype. The most important of these 

 are the retractor muscles, which, arising from the bottom 

 of the cell, in the form of long, somewhat flattened, 

 transversely striped, isolated fibres, about the one ten- 

 thousandth of an inch in width, are inserted, some of them 



