Technical Note.—Some common water-leeches, alive or preserved in alcohol, should be examined by the class. The animals are not unfamiliar to boys who "go in swimming" in the small streams of the country. The body of a leech should be examined carefully, and drawings of it showing the external structural characters should be made.

The body of a leech is flattened dorso-ventrally, instead of being cylindrical as in the earthworm, and tapers at both ends. In the live animal the body can be greatly elongated and narrowed or much shortened and broadened. It is composed of many segments (not as many as there are cross-lines however; each segment is transversely annulated), and bears at each end on the ventral surface a sucker, the one at the posterior end being the larger. These suckers enable the leech to cling firmly to other animals. The mouth is at the front end of the body on the ventral surface and is provided with sharp jaws. Leeches live mostly on the blood of other animals which they suck from the body. The common leech "fastens itself upon its victim by means of its suckers, then cuts the skin, fastens its oral sucker over the wound and pumps away until it has completely gorged itself with blood, distending enormously its elastic body, when it loosens its hold and drops off." Its biting and sucking cause very little pain, and in olden days physicians used the leeches when they wanted to "bleed" a person. A common European species of leech much used for this purpose is known as the "medicinal leech." All leeches are hermaphroditic, that is, the sexes are not distinct, but each individual produces both sperm-cells and egg-cells. Most of the leeches lay their eggs in small packets or cocoons. This cocoon is dropped in soil on the banks of a pond or stream so that the young may have a moist but not too wet environment. The young issue from the eggs in four or five weeks, but they grow very slowly and it is several years before they attain their full size. Leeches are long-lived animals, some being said to live for twenty years.

Flatworms (Platyhelminthes).—Technical Note.—Collect some live fresh-water planarians (see fig. [30]), which are to be found on the muddy bottom of most fresh-water ponds, and examine them while alive in watch-glasses of water. Make drawings showing the external appearance, and as much of the internal anatomy as can be seen. The branching alimentary canal can be seen in more or less detail, and with higher power of the microscope parts of the nervous system can be seen also. Have also a tapeworm preserved in alcohol or formalin to show the very flat and many-segmented body.

The flatworms include a large number of forms which vary much in shape and habits. They are all, however, characteristically flat; in some this condition is very marked. Some are active free-living animals, as the planarians (figs. [30] and [31]), while many live as parasites in the alimentary canal of other animals, as do the sheep-fluke and the tapeworms.

Fig. 30.—A fresh water planarian, Planaria sp. (From a living specimen.)

The fresh-water planarians (fig. [30]), which live commonly in the mud of the bottom of ponds, are small, being less than half an inch long. They are very thin and rather broad, tapering from in front backwards. On the upper surface near the front they have a pair of eyes; the mouth is on the under surface a little behind the middle of the body. The alimentary canal is composed of three main branches, each with numerous small side branches. One main branch runs forward from the mouth, and the other two run backwards, one on each side of the body. There is no anal opening, and the alimentary canal thus forms a system of fine branches closed at the tips, and extending all through the body. The nervous system is composed of a ganglion or brain in the front end of the body from which two main branches extend back throughout its whole length. From these main longitudinal branches arise many fine lateral branches.