POLYZOA. 527 



Alsinastrum, and finding it grew freely, but soon covered 

 with a filamentous confervoid growth, threw in two small 

 water-snails, which are there still. About January last, 

 the ova, which till then had floated on the surface of the 

 water, began to sink and attach themselves to the leaves of 

 the Anacharis and elsewhere. Latterly, they have all 

 subsided to the bottom of the jar, where they lie in com- 

 pany with a quantity of decayed vegetable matter, spawn 

 of the Limnceus, &c. They are of a light-brown colour, 

 ovoid in shape, longest diameter -0089, shortest diameter 

 0172. The outer rim seems built up of cells of oblong 

 shape, but necessarily ill-defined, owing to their being ob- 

 served by light transmitted through two surfaces; the 

 inner or central portion also cellular, but from the con- 

 vexity of the object, more easy to determine as to its true 

 nature, formed of larger hexagonal-shaped cells. Seen by 

 higher power (J-in. obj. A. eye-piece 220 lin. diam.), 

 these central cells, besides being unmistakably hexagonal 

 in form, have each a distinct dark nucleus in the centre : 

 this, however, may be an optical fallacy, due to their 

 peculiar position on a curved surface. No movement yet 

 visible, April 25, 1857." 



Plumatella fiepens, Plate JY. No. 99, so named from its 

 feather-like crown of tentacles, is a well-known fresh- water 

 Polyzoon, found in ponds and rivulets attached to aquatic 

 plants, generally choosing the under-surface for the pur- 

 pose of avoiding the strong light. It is a very elegant 

 variety, rather timid, withdrawing on the least disturbance 

 of the water, and not again venturing to display its beau- 

 tiful plume until all is once more perfectly quiet. Pro- 

 fessor Allman says of it : " Except in the condition of 

 the dermal system the structure of Plumatella differs in 

 no essential point from that of A Icyonella. This system, 

 however, in the coalescence of the tubes into a common 

 mass in Alcyonella, while they remain totally distinct in 

 Plumatella, presents us with a difference of sufficient im- 

 portance to justify the placing the two forms in separate 

 generic groups. The number of known species are twelve, 

 of which nine are Britiph. The csensecium consists of a 

 linear-branched series of tabula, cells of membrano- 

 corneous consistence, which is terminated by the orifice 



