Page:Natural History, Mollusca.djvu/329



The stones that are ordinarily covered by the tide, and the coarser sea-weeds, such as the Fuci and Laminariœ, that grow at low-water, are very frequently studded with irregular patches of dark-coloured substance, gelatinous to the feel, and often somewhat brightly coloured, the more common hues being blue, purple, green, grey, and white. On closer examination, we find embedded in this mass, circles of stars, each consisting of a definite number of bright-hued, minute, oblong bodies, radiating from a common centre.

These masses belong to the genus Botryllus, the representative of the family before us; and each radiating point is an individual animal. From ten to twenty of such animals are ordinarily grouped together to form one of the wheel-like systems; and there are often as many systems in one encrusting mass.

The organization of these little animals is in general conformable to what has been already described; but the discharging orifice of each individual is placed at the opposite end from the mouth, and opens into the common centre, which, rising with a circular rim, expands and contracts as a discharging orifice for the whole of that system.

Several species of this genus are common with us, one of the most abundant of which is the Botryllus polycyclus, which is found encrusting the broad leaves of the common Tangle (Laminaria digitata). It is of a bluish purple hue, with the individuals marked by white rays.