ALCYONIDIUM MYTILT. 499 



land (Dalyell) : South Devon, Salcombe Bay, Torbay, 

 incrusting shells of Trochus, &e. ; Mount's Bay, Corn- 

 Avall ; Ilfracombe ; Llandudno ; Isle of Man, common ; 

 Menai Straits ; Herm, on Trochus (T. H.) : Northumber- 

 land, common (Alder). 



Geographical Distribution. Bahusia, on Algffi, 5-20 

 fathoms (Sniitt) : Baltic Sea, eastern basin, very abun- 

 dant (K. Mobius). 



The cells in this species are normally hexagonal ; but 

 they are subject to many irregularities, and in older 

 colonies, where they are closely packed together, they vary 

 greatly in figure. The septa are usually distinctly visible 

 on the surface. The crust is sometimes very thin ; in 

 other states it is much thicker, loses much of its trans- 

 parency, and becomes charged (according to Smitt) with 

 argillaceous particles. There are also varieties in the 

 colour. The species, however, is readily recognizable by 

 its comparatively smooth surface, mapped out into distinct 

 areas by the zooecial septa. 



The ovaries are distributed irregularly, and often in 

 great numbers, over the zoarium, and occur as somewhat 

 circular papillae, within which the opaque-white ova are 

 visible, ranged so as to form a ring. The ovarian cham- 

 ber is merely a zooecium destitute of polypide, and set 

 apart for the discharge of reproductive functions. 



The mature embryos may be seen moving about freely 

 within the cavity ; when about to escape, a tubular orifice 

 is gradually pushed forth at a point near the top of the 

 cell, marked by a slight depression, through which they 

 successively work their way by means of their cilia. I 

 have seen seven pass from a single chamber in the course 

 of a few seconds ; and very interesting it was to watch 

 them struggling through the narrow passage and launching 



