The body is generally depressed or flattened, but convex above, and is composed of seven segments, each segment bearing a pair of legs which terminate in a pointed claw, while the posterior appendages are modified into flat, leaf-like organs of respiration.
Fig. 199.—Marine Isopod
1. Sphæroma serratum. 2. Limnoria lignorum. 3. Ligia oceanica. 4. Nesæa bidentata. 5. Oniscoda maculosa
When engaged in ‘shrimping’ one frequently meets with shrimps or prawns that are disfigured by a tumourous swelling on the side of the body, and if the swelling be opened a little parasite will be dislodged. This parasite is an Isopod (Bopyrus), the appendages of which are imperfectly developed. The female is very much larger than the male, and, as is usual with parasitic creatures, the greater part of the body-cavity is occupied by the well-developed organs of reproduction.
There are several other parasitic isopods, some of which live on the bodies of fishes, and are popularly known as fish-lice, but these are not so likely to come in the way of the sea-side naturalist as the more typical forms that crawl about on the rocks and among the weeds of the coast. A few of the latter are shown in the accompanying illustration, including the Sea Pill-ball (Nesæa bidentata), common on the rocky coasts of the south-west, and distinguished by the two sharp projections on the last segment; the Serrated Pill-ball (Sphæroma serratum), very common on most rocky shores, and characterised by the fine sawlike teeth on the outer edge of the outer plates of the ‘tail’; the Great Sea-slater (Ligia oceanica), also an abundant species; the Spotted Hog Louse (Oniscoda maculosa) that lives among the tufted sea weeds; and the Boring Pill-ball (Limnoria lignorum) that bores into the woodwork of piers and jetties, often doing considerable damage.
Fig. 200.—Marine Amphipods