ZOOPHYTES. 247 



thither for a serious infirmity, in early youth, I recollect the natural 

 productions likely to stimulate my juvenile curiosity, which were dis- 

 played in the threshold of Dr Walker's dwelling. The Chair of Natural 

 History, which is among the later academical institutions in the Univer- 

 sity of Edinburgh, becoming vacant, Dr Walker was appointed to fill it. 

 On his demise, the present worthy and learned Professor Jameson, whose 

 useful labours have contributed so effectually towards the advancement 

 of science in Scotland, was chosen to succeed him. 



The Valkeria imbricata rises erect, about two inches in height, by a 

 short stem, and then subdivides into slender boughs, waving slightly, with 

 branches in alternate arrangement on their opposite sides, originating 

 from the convexities. — PL L. fig. 1. 



Masses or clusters, composed of a double row of cells, wind spirally 

 up the surface of the skeleton, at irregular intervals. This peculiar order 

 is distinct in their earlier stages, but with the increment and accumulation 

 of cells, the spiral progress, and the disposal of the cells in pairs, become 

 less evident ; it is gradually obscured, and finally lost. — Fig. 2. 



The cell, of ovate form, is occupied by an ascidian hydra, with eight 

 ciliatsd tentacula. The cilia are stout, and very conspicuous, as their 

 successive action is exhibited, fig. 3. 



As the cell is connected with the body of the ascidian hydra, whether 

 by ligaments below, or by the union of the margin above, it undergoes a 

 great modification with the position of its tenant. When the latter is 

 displayed, the cell is extended : when in retreat doubled on itself, the cell 

 becomes ovate. 



The stem seems tubular. Its extremities are obtuse. But neither 

 the surface of the stem nor that of the cell is smooth. 



Finer specimens exhibit numerous subordinate parts, of which the 

 most remarkable are the clusters of cells. One an inch higli, bore be- 

 tween 60 and 70 such clusters, with their hydra;. Another, an inch and 

 three quarters high, had 19 clusters on a single branch. The former spe- 

 cimen spread about an inch. But, in studying the subject in detail, it is 

 better to select more meagre specimens, where the multiplicity of parts 

 does not intercept the view. 



