_C. refracta_ grows in dense tufts, two or three inches long, in rock pools near low-water mark. Its fronds consist of rigid stems in rope-like bundles that are very freely branched, the whole tuft being of a yellow-green colour and silky texture. _C. albida_ closely resembles it in structure and habit, but may be distinguished by its paler colour, which disappears when the weed is dried, and by its longer and more delicate branches. In another order of the green-spored algæ (the _Siphoneæ_ or _Siphonaceæ_) the frond is formed of single branching cells, and many of these are often interwoven into a spongy mass, and sometimes coated with a deposit of calcareous matter. In the genus _Codium_ the fronds are of a sponge-like texture, composed of interwoven branching fibres, and are of a globular, cylindrical, or flattened form. The commonest species is _C. tomentosum_ (Plate VII.), which consists of sponge-like, dark-green cylindrical fronds, which are forked and covered with short hairs that give it a woolly appearance when in the water. Each frond is composed of slender interwoven fibres with club-shaped filaments passing vertically to the surface. It grows on rocks in the pools between the tide-marks, and is abundant on nearly all our coasts. The Purse Codium (_C. bursa_) has spongy hollow fronds of a globular form, varying from a quarter of an inch to five or six inches in diameter. It is a rare species, being found only at a few places on the south coast. Another species (_C. adhærens_) adheres to rocks, over which the fronds spread in irregular soft patches, the club-shaped vertical filaments of its interwoven fibres giving it the appearance of rich green velvet. An allied weed (_Bryopsis_), named from its moss-like appearance, grows in erect tufts, each frond consisting of a branched one-celled filament. There are two species of the genus, one (_B. plumosa_) characterised by the light feathery nature of its fronds, the stems of which are branched only near the top. It is found in rock pools on most of our coasts. The other (_B. hypnoides_) is more freely branched, and the branches are long, and issue from all sides of the stem. Like the last species, it has branches on the outer part of the stem only, but it is of a softer texture.