CRISTATELLA. 89 



In general description these three stand as follows : — 



I. The Cristatella, consisting of a multitude of independent hydra?, 

 rising as a border from a gelatinous or fleshy sole or base, which is en- 

 dowed with a slow locomotive faculty. 



II. The Alcyonella, consisting of a multitude of hydrae, rising prornis- 

 culously from a gelatinous mass or stratum, serving as a polyparium, which 

 invests some foreign substance in firm and permanent adhesion. 



III. The Phtmatella, consisting of a vegetation in arborescent form — 

 stem and branches wholly tubular, and, for the most part, adhering later- 

 ally to indurated or vegetable substances, the hydrae issuing from the ex- 

 tremities. 



All these products are perpetuated by capsules or ova. 



§ 1. Cristatella vagans. — The Erratic Cristatella.* — Plates XXVII., 

 XXVIII. — Among the zoophytes of the fresh-waters of Scotland this is, 

 perhaps, the most remarkable of all. The features, by which it is dis- 

 tinguished, belong to none of the rest, nor, it may be, to any other known 

 animal of the universe. 



The erratic Cristatella is of various, probably of indefinite dimensions : 

 in highest luxuriance exceeding two inches in length, by three or four 

 lines in breadth. It is of a flattened form, with straight sides and obtuse 

 extremities ; or the sides are sometimes slightly curved, the body elliptical, 

 and, considering the convexity of the surface, it rises equal to a longi- 

 tudinal section of the fourth of a cylinder. The middle of the back is 

 bare, and bordered by a triple row of about 350 erect, single, ascidian 



* Perhaps Lunatella, as indicating the form and arrangement of some of the external 

 parts, might be a more suitable name than Cristatella. 



While labouring under considerable embarrassment, as I do still, regarding the identity 

 of the genera described in this chapter, with those of other observers, I submitted a few re- 

 marks on the subject to the British Association, when assembled in Edinburgh during the 

 year 1834. This I did principally for the purpose of inviting the attention of Scotish 

 naturalists to an interesting field for investigation, whereon I was not aware, that any of 

 them had bestowed special notice. 



VOL. II. M 



