On the Llano Estacado there are both freshwater and salt lakes, and there are a few salt lakes in the Trans-Pecos Province and near the mouth of the Rio Grande on the Coastal Plain.
The very large assemblage of forms coming under this order comprises the most highly developed predaceous sea-snails, numerous vegetarian species, a considerable number of freshwater and some terrestrial forms. The partial dissection of a male specimen of the common periwinkle, Littorina littoralis, drawn in fig.
A very fine freshwater fish is the Murray cod, which sometimes weighs Too lb; and the golden perch, found in the same river, has rare beauty of colour.
Several of the marine and many species of freshwater algae are peculiar to the island.
In freshwater lakes and ponds, especially if the water is stagnant, aquatic plants are abundant.