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tains an ascidian polype, which has about twelve tentacula and 

 no gizzard. The polype, so far as I have observed, is always lodged 

 in the upright portion of the cell, but the long retractor muscular 

 fibres arise near the commencement of the horizontal portion of the 

 cell, and from its upper wall, nearly at one point. They consequently 

 turn abruptly round the angle formed by the rising up of the cell, and 

 passing chiefly on the side (the anterior) of the body of the polype, 

 are inserted, some towards its inferior part, and some apparently near 

 the base of the tentacula, as in other polypes of this class (fig. 7). 

 The muscular fibres consequently are of great proportionate length ; 

 they are distinctly marked with transverse striae, and appear to me to 

 be constituted of a delicate diaphanous tube, or sarcolemma, contain- 

 ing the more solid, contractile, sarcous particles. I am led to this 

 conclusion, from observing here and there in some of the fibres, 

 spaces in which the fibre is very transparent and unmarked with striae, 

 as if a tube were stretched and two portions of its contents drawn 

 apart within it (fig. 8). The tentacula are from ten to twelve in 

 number ; they are very long, and have a series of spinous projections, 

 as remarked, I believe, by Mr. Lister, on their outer side. The re- 

 semblance of the terminal expanded part of the polype cell to the 

 head of a snake is very striking, and in fact, of the whole cell to the 

 body of a serpent, and thence has arisen its very appropriate generic 

 name. It would appear, from the descriptions heretofore given of this 

 zoophyte, that the form and situation of the aperture have not been 

 rightly apprehended ; and as in the systematic arrangement of this 

 class, these particulars will be found of the first importance, it is to 

 this part I would direct attention. The aperture is terminal, and in 

 form crescentic (fig 2), the lower lip being formed of, or strength- 

 ened by a cartilaginous or horny half-ring, very nearly resembling a 

 horseshoe in form, and which constitutes the movable part of the 

 rim of the orifice, and to which are attached two pairs of mus- 

 cles, one for the closure, and the other for the opening of the aper- 

 ture. I should have stated, in speaking of the structure of the 

 walls of the polype cell, that the upper or dorsal portions only 

 of the expanded extremity were strengthened by calcareous matter, 

 the lower side being filled up by a thin flexible membrane, attached 

 in front to the cartilage of the lower lip of the aperture, and which 

 by its flexibility, allows the latter to be moved with ease. This mem- 

 branous expansion may be likened to the bag of the pelican's beak, 

 and like it, seems sometimes to be used as a receptacle for food, at all 

 events, it will occasionally be found to contain numbers of Naviculce 



TRANS. MIC. SOC. VOL. II. M 



