ZOOPHYTA ASCIDIOIDA. 239 



tween the polypidoms there is an apparent affinity. The Crisiadoe 

 are not unhkc the Sertuhiriacla?, and it is still disputed whether 

 some Gemmicellariyo appertain to this family or to the Flustra ; 

 the resemblance between the Sertularite and the Vesiculariadse 

 misled even Lamarck to their union under one genus ; and their 

 names would seem to imply that the framers of the genera Al- 

 cyonidium and Alcyonella believed them to be in a family re- 

 lationship to Alcyonium. These are examples which prove the 

 fallacy of outward characters ; and how darkly the naturalist 

 must grope his way who would walk through Nature's labyrinth 

 without the ariadnsean thread that the anatomist alone can give 

 him ! In the instance before us he has demonstrated that the 

 resemblances indicated above imply no propinquity in their ob- 

 jects. The ascidian polypes the Creator has cast in the mould 

 — not of the radiata — but of the mollusca, — yet with such a con- 

 siderable variation as to mark their ordinal distinctness ; for the 

 Mollusca Tunicata — their nearest allies — are not protrusive from 

 their cells as these polypes are ; and this seeming slight dis- 

 crepancy connects itself, perhaps of necessity, with a total change 

 in the position and nature of their respiratory organs. Interior 

 and immotive in the one tribe, they line, in a reticular pattern, the 

 parietes of a sac capacious enough to contain a sufficiency of the 

 aerating fluid ; while in the other they clothe the exsertile ten- 

 tacula in the form of cilia which must be placed outwards amid 

 the circumfluent waters before they can play and fulfil their func- 

 tions. 



Notwithstanding the great diversity in the forms of the poly- 

 pidoms of this order, there appears to be a very remarkable uni- 

 formity in the habit and structure of the polypes. The body lies 

 doubled up in the cell (Fig. 39, a, b.), its oral extremity crowned 

 with a circle of lono- filiform ciliated tentacula.* From the 

 centre of this circle the mouth opens into a sort of pharynx 

 (Fig. 39, a, 1.) which begins the cesophageal tube, generally of 



* At page 34 the tentacula of the Ascidioida are stated to be solid .• an error 

 which Dr Farre enables us to correct. In Alcyouidium, " with an amplifying 

 power of 200 linear," " they are seen to be (ubular throughout, and to have an 

 aperture at each extremity; (hat nt the base apparently communicating with a 

 fine canal round the oral rim, which probably unites the tentacular canals into 

 one circle." Phil. Trans, an. 1837, p- 406. This is a structure very like that 

 in the two preceding orders. 



