Other species are characterised by transparent internodes as above described, but have no spines at the joints, and may thus be easily floated on to a sheet of paper without the troublesome matting of their fronds. These include the Straight Ceramium (_C. strictum_), with erect and straight branches growing in dense tufts, and conspicuous tetraspores arranged round the nodes of the upper branchlets, _C. gracillimum_, of the lower rock pools, with very slender gelatinous fronds, swollen nodes and small fan-shaped branchlets; _C. tenuissimum_, closely resembling _C. strictum_ in general appearance, but distinguished by having its tetraspores only on the outer side of the nodes; and the Transparent Ceramium (_C. diaphanum_), which may be found throughout the year on rocks and weeds in the rock pools. The last species is the largest and most beautiful of the genus, and may be readily recognised by its light-coloured, transparent stem with swollen purple nodes, and its conspicuous spore-clusters near the tips of the filaments. [Illustration: FIG. 250.--_Ceramium diaphanum_] Our last example of the genus is the Common Red Ceramium (_C. rubrum_), which may be found in the rock pools at all levels. It is very variable in form, but may be known by its contracted nodes, in which the red tetraspores are lodged, and its spore-clusters surrounded by three or four short branchlets. It differs from most of the other species in having both nodes and internodes covered with cortex-cells, and hence the latter are not transparent. The order _Spyridiaceæ_ has a single British representative which may be found in various localities on the south coast. It is _Spyridia filamentosa_, a dull-red weed with thread-like, tubular, jointed fronds, from four inches to a foot in length. The main stem is forked, and densely clothed with short and slender branchlets. The frond is covered with a cortex of small cells. The spore-clusters are grouped together, several being enclosed in a membranous cell in conceptacles, or external sacs, at the ends of the branchlets; and the tetraspores are arranged singly along the jointed branchlets. The next family (_Cryptonemiaceæ_) is an extensive one, containing nearly twenty British genera of red or purple weeds, with unjointed, cartilaginous, gelatinous, and sometimes membranous fronds. The spores are irregularly distributed, and are contained either in sunken cells or in conceptacles. The tetraspores are either in cells at the edges of the frond or collected together in compact groups.