Even those crustaceans that are protected by a hard, calcareous exo-skeleton, and the molluscs that cannot be removed from their stony shells without injury to their soft structures, may be studied in the manner just described, and this may be done by first soaking them in dilute hydrochloric acid, renewed as often as may be necessary, until all the mineral matter has been dissolved completely, and then hardening the softer tissues in one of the reagents mentioned above. Hydrochloric acid may also be used to dissolve the calcareous shells of foraminifers, the vegetable corallines, and other small forms of life, previous to microscopic examination of the soft parts. CHAPTER VII _THE PROTOZOA OF THE SEA SHORE_ We shall now study the principal forms of animal life to be found on the sea shore; and, in order that the reader may thoroughly understand the broader principles of classification, so as to be able to locate each creature observed in its approximate position in the scale of life, we shall consider each group in its zoological order, commencing with the lowest forms, and noting, as we proceed, the distinguishing characteristics of each division. The present chapter will be devoted to the _Protozoa_--the sub-kingdom that includes the simplest of all animal beings. Each animal in this division consists of a minute mass of a jelly-like substance called _protoplasm_, exhibiting little or no differentiation in structure. There is no true body-cavity, no special organs for the performance of distinct functions, and no nervous system. Perhaps we can best understand the nature of a protozoon by selecting and examining a typical example: Remove a small quantity of the green thread-like algous weed so commonly seen attached to the banks of both fresh and salt water pools, or surrounding floating objects, and place it in a glass with a little of the water in which it grew. This weed probably shelters numerous protozoons, among which we are almost sure to find some _amœbæ_ if we examine a drop of the water under the high power of a microscope. [Illustration: FIG. 50.--THE AMŒBA, HIGHLY MAGNIFIED] The amœba is observed to be a minute mass of protoplasm with an average diameter of about one-hundredth of an inch, endowed with a power of motion and locomotion. Its body is not uniformly clear, for the interior portion is seen to contain a number of minute granules, representing the undigested portions of the animal’s food. There is a small mass of denser protoplasm near the centre, termed the _nucleus_, and also a clear space filled with fluid. This latter is called the _vacuole_, and is probably connected with the processes of respiration and excretion, for it may be seen to contract at irregular intervals, and occasionally to collapse and expel its contents.