We may well begin our study of the seaweeds with an examination of the external structure of as many different kinds as we can find. Some of them are flattened and very thin forms and of them the Sea Lettuce may be taken as typical. This plant, known to scientists as _Ulva Lactuca_, occurs at high-water mark. In its fully developed form it is pale green and so thin as to be almost transparent; its structure may be studied under the microscope without difficulty. Then there is the very common, green, Compressed Enteromorpha which grows in great profusion on the rocks of the shore, rendering them exceedingly slippery. The closely related Intestinal Enteromorpha as it floats in the water resembles a green, membranous tube and those of us who have ever done any zoological dissection will appreciate how well named this plant is. The structure of both the Enteromorphas can easily be seen. Many of the brown and red _Algæ_ will provide us with a good deal of occupation in making out their structure. Some of them, the brown _Ectocarpus Siliculosus_ for example, may be found, growing in moss like tufts, which are usually attached to one of the larger _Algæ_ living between the tide marks. It is one of the simplest of the brown sea weeds, consisting of branched threads, but one cell in thickness. The Wracks, of which the Common Bladder Wrack or Pop weed with its little air filled bladders is familiar to everyone, are more complicated in structure, in fact they appear to be possessed of stems and leaves, but we shall return to them in a moment. Most of the common red _Algæ_ are so delicate in structure that they require a fairly high magnification for their examination. The thin, membranous fronds of the beautiful crimson _Delessera Sanguinea_, may be sought below low tide limit or may be found washed up upon the shore. Superficially the plant resembles a red hart’s-tongue fern, with much more delicate fronds than ever fern of that species possessed. We may well compare its structure with that of the sea lettuce, for it is equally transparent. In the rock pools of many parts of the coast we may happen upon a most curious almost white sea weed, known as Coralline or _Corallina Officinalis_. Its branched, feathery stems are hard and stony and the whole plant bears a superficial resemblance to a coral, hence its name. The plant absorbs a substance known as calcium carbonate from the sea water and deposits it in the form of a hard, stony covering over its surface. Calcium carbonate does not occur in sea water everywhere, at least not in sufficient quantity to be of use to the Coralline, that is the reason the sea weed is not quite so common as some of the others we have mentioned. The curious armour-plating of this sea dweller, should be studied under the microscope.