VESICULARIADJ: : BOWERBANKIA. 379 



tracts very rapidly." Dr. Coldstream. According to Mr. J. V. Thomp- 

 son this species creeps over the surface of the Fuci by means of its 

 tubular ramifying roots, and throws off numerous flaccid irregularly 

 branched shoots to the length of from one inch to one and a half or 

 more, often so densely clustered as entirely to cover the plant on 

 which it grows. 



" The animal when fully expanded is about one-twelfth of an inch 

 in length. In its retracted state it is completely inclosed in a deli- 

 cate horny cell, sufficiently transparent to admit of the whole struc- 

 ture of the contained animal being seen through its parietes. The 

 cells are connected together by a cylindrical creeping stem, upon 

 which they are thickly set, and sessile, ascending from its sides and 

 upper surface." 



" The animal when completely expanded is seen to possess ten 

 arms of about one-third the length of the whole body, each arm being 

 thickly ciliated on either side, and armed at the back by about a 

 dozen fine hair-like processes, which project at nearly right angles 

 from the tentacula, remaining motionless, while the cilia are in con- 

 stant and active vibration." Farre. 



B. " imbricata, in the first stage of its formation, consists of a 

 single layer of cells spread over the surface to which it is attached 

 (usually Fucus vesiculosiis), and not rising from it in the form of an 

 independent polypidom. In this stage of its growth it constitutes 

 the Bowerbankia densa of Dr. Farre. This fact I have ascertained 

 from a comparison of Dr. Farre's figure and description of that spe- 

 cies with it, and its concurrence with these is so close as not to admit 

 of a doubt upon the subject. Bowerbankia densa is, therefore, not a 

 distinct species, but merely a condition of the well-known one, Val- 

 keria imbricata. Although the examination of numerous specimens 

 of V. imbricata which I have made has resulted in the eradication of 

 B. densa as a distinct species, I yet must not omit to notice the ad- 

 mirable memoir published in the ' Philosophical Transactions,' upon 

 this and an allied species by Dr. Farre, the gentleman by whom 

 Bowerbankia densa was first described and figured as a distinct spe- 

 cies, and to whom we are indebted for almost all we know of the ana- 

 tomy of the Ascidian type of zoophytes. 



" Some time since I forwarded to Dr. Johnston specimens under the 

 name of Bowerbankia densa for examination ; one of them was in fact 

 Valkeria imbricata in the primary stage of its growth, that is, 

 spreading over a plain surface ; the other was elevated in the form 

 of a distinct polypidom, the condition in which V. imbricata is 



