web a funnel-like tube runs downward and inward. In this tube the spider spends most of the time, running out to catch insects which may become entrapped. At the lower end of the tube is an opening through which the spider may escape in time of necessity. The funnel-web builders are strong-legged, active spiders.

The orb-weaving spiders spin webs of geometrical exactness in bushes or long grass. They are usually of almost circular form with a spirally wound center thread supported on guy lines which are attached firmly to surrounding objects. These webs, which act both as homes and snares, are made of two kinds of silk, a supporting thread, tough but rather inelastic, and a thinner elastic sticky thread, out of which the snare is woven. The outer part of the web forms the snare. The central part of the web usually contains a shield of closely woven silk on which the spider may rest. Some orb weavers live near one edge of the web, hanging suspended within easy reach of a possible capture. In traveling over the outer part of the web the spider uses the guy lines only, as otherwise it might destroy its own web. Why?

One of the commonest of the orb weavers is a large yellow and black spider known as Argiope. Their webs may be found in almost any garden or yard.

Find such a web. Describe its location. How is it attached? How many guy lines does it contain? Look for the central shield on which the

spider rests. Do you find a "winding stair"? Notice the open area between the central home of the spider and the outer or spiral zone of the web. This area, known as the free zone, gives opportunity for free movement around the web, as the spider does not travel on the sticky outer portion.

Other Forms of Web. - Other forms of webs are seen in the wonderful nest of the trapdoor spider which, after excavating a hole in the ground, lines it with silk and then makes a lid of earth also lined with silk. This lid is closed by the spider after its retreat to the hole. Other spiders use the web for bridge building. In this case a long single strand is spun which is allowed to float off behind the spider into the air. This is flown like a

kite until it catches some projection, when the spider hauls in the slack, makes it fast, and travels across on the slender bridge thus built.

Myriapods. We are all familiar with the harmless and common thousand legs found under stones and logs. It is a representative of the group of animals known as the millepeds. These animals have the body divided into two regions, head and trunk. They have two pairs of legs for each body segment. The centipeds, on the other hand, have only one pair of legs to each segment. None of the forms in the eastern part of the United States are poisonous.

CLASSIFICATION OF ARTHROPODA

CLASS, Crustacea. Arthropods with limy and chitinous exoskeleton, breathing by gills, and having two pairs of antennæ.

SUBCLASS I. Entomostraca. Crustacea with a variable number of segments, chiefly small forms with simple appendages. Some degenerate or parasitic. Examples, barnacles, water flea (Daphnia), and copepods (Cyclops). SUBCLASS II. Malacostraca. Usually large crustacea having nineteen pairs of appendages. Examples, American lobster (Homarus americanus), crab (Cancer), or shrimp (Palamonetes).

CLASS, Hexapoda (insects). Arthropoda having chitinous exoskeleton, breathing by air tubes (tracheœ), and having three distinct body regions.

Order, Aptera (without wings). Several wingless forms. Examples, springtails. Order, Orthoptera (straight wings). Example, Rocky Mountain locust.

Order, Lepidoptera (scale wings). Examples, cabbage butterfly, cecropia moth. Order, Diptera (two wings). Examples, house fly, mosquito.

Order, Hemiptera (half wing).

Order, Neuroptera (nerve wings).
Order, Coleoptera (shield wings).

Examples, all true bugs, plant lice, and cicada.
Examples, May fly, dragon fly.
Examples, beetles.

Order, Hymenoptera (membrane wings). Examples, bees, wasps, ants.
CLASS, Arachnida. Arthropoda with head and thorax fused. Six pairs of
appendages. No antennæ. Breathing by both lungs (spiders) or trachea.
Examples, spiders and scorpions.

CLASS, Myriapoda. Arthropoda, having long bodies with many segments; one or two pairs of appendages to each segment. Breathing by means of tracheæ. Example, centiped.

REFERENCE BOOKS

FOR THE PUPIL

Needham, Outdoor Studies.

American Book Company.

FOR THE TEACHER

Emerton, The Structure and Habits of Spiders. Knight and Millet.

XXI. MOLLUSKS

=

Mollusca. The name Mollusca (Latin mollis soft) gives the character which chiefly aids us in identifying a mollusk. The body is soft and unsegmented. It is usually covered with a limy shell, formed by the agency of a delicate envelope called the mantle. The animal usually possesses a single muscular foot, by means of which locomotion takes place. There are several groups of mollusks which are, as we shall see, quite unlike in appearance and in habits.

Notice that the

The Shell of the Fresh-water Mussel (Unio species).1 shell is made up of two parts or valves. Such a shell is called a bivalve. Notice that the valves are joined together by a structure, somewhat elastic, called the hinge ligament. Close the two shells; why do the shells spring open again? The lines which run more or less parallel to the edge of the shell are called lines of growth. If a line of growth once represented the outer edge of the shell, then

find the oldest part of the shell. This raised area is called the umbo. It is always possible to locate the anterior end of the shell because the umbo points toward that end. The hinge ligament marks the dorsal side of the animal.

The shell is covered on the outside by a thin layer of horny material. This is called

the periostracum. Can you explain why it does not cover the entire shell? The shell proper, if tested with acid, will be found to contain considerable lime.

Shell of fresh-water clam, the left half polished to show the prismatic layer.

Draw, natural size, a single valve, and locate the hinge ligament, umbo, and lines of growth. Place the dorsal surface upmost in the drawing.

Mussels may be opened by first placing the living animal in hot water until the shell gapes. Then insert a knife, keeping the blade close to the inner surface of one valve, cut through the tough muscles which hold the shells together, and the shells will open. Notice the mother of pearl covering the inner surface. Notice in a freshly opened clam that a delicate membrane, the mantle, adheres to the shell.

Structure of Shell.-The shell, if examined in cross section with a good lens, is seen to be made up of three layers: the outer periostracum, made

1 See Hunter and Valentine, Manual, page 138, for exercise on Venus Mercenaria.

C

P

L

up of material much like that which forms a cow's horn, or chitin; a middle layer composed of tiny prisms of lime held in by the horny material (this S layer is called prismatic layer); and an inner layer (the laminated layer), made up of layers of lime and horn alternating parallel to the surface of the shell. The inner layer is formed by the action of the whole surface of the mantle. The two outer layers, are made by the edge of the mantle only. So a shell grows in thickness largely from the inner surface of the mantle, while it grows in diameter from the edge of the mantle only.

M

Vertical section of shell and mantle of a mollusk; C, periostracum; P, prismatic layer; L, laminated layer; S, shell; M, mantle. (After Claus.)

The Open Shell. - Pull the shells completely open. Find on the dorsal side projections and grooves which fit into each other when the shell is closed. These are the hinge teeth. Compare the number in each shell. How might they be of use to the animal? Find the marks on the shell where the adductor muscles were fastened. What was the use of the adductor muscles? Why do dead mussels always have the shell partly open?

Draw one opened valve showing all above parts. Label the anterior and posterior adductor muscle scars, according to position.

d

Body and Mantle Cavity. - In one valve lies the body of the clam. If we remove the mantle, we shall find under a roundish soft mass, the body, or visceral mass. Surrounding the visceral mass but ventral to it is a cavity bounded on the outside by the inner surface of the mantle. This is the mantle cavity. In life this cavity is full of water. See if you can discover how and where water gets in. In a living mussel the posterior edge of the mantle on the right side is folded so as to fit with the adjoining edge of the mantle on the left side. The funnel-like openings thus formed are called siphons.

If

Siphons. The siphons can best be seen in living mussels which have been left quiet for some time in an open trough or tank. a little powdered carmine is allowed to drop from a medicine dropper close to the siphons (the fringed edges of which may be seen extending from the shell), a current of water will be seen to draw in and expel the carmine grains. Where is the incurrent siphon with reference to the excurrent? (In the "long-necked" or "soft" clam the siphons are greatly developed and are made of

a

B

a

Cross section of a mollusk; A, mantle cavity; a, shell; b, gills; B, cloacal cavity; k, body.

tough muscles. If they are cut lengthwise, the two tubes, incurrent and excurrent, can be easily seen.) The siphon permits water, bearing food and oxygen, to get into the mantle cavity. Here are found the gills.

The Gills. -The gills are striated platelike structures lying on each side of the visceral mass. How many gills on each side? Any difference in size of those on one side? When the clam is in a natural position, the gills hang freely in the mantle cavity. In structure each gill is a long, narrow bag open on the dorsal side. This baglike opening leads into a second cavity, dorsal to the mantle cavity. This space, called the cloacal cavity, is in communication with the outside through the excurrent siphon. A mussel when viewed from one end or in cross section somewhat resembles a book. The shell has the position of the board cover, the mantle the paper pasted to its inner surface, the gills the fly leaves, and the body the printed pages in the book.

A fresh-water mussel with the right valve, mantle, and gills and some tissue at the base of the foot removed; a, anterior adductor muscle; au, auricle of the heart; ft, foot; g, reproductive gland; gl, gill; i, intestine; k, kidney; m, mantle; n, nerve ganglia connected by nerve cords o; p, posterior adductor; pl, labial palps; si, siphon; v, ventricle. The dark lines on the mantle and foot are blood vessels. Davison, Zoology.

Circulation of Water over Gills. We have already observed that a more or less constant circulation of water takes place; carmine entering through the incurrent siphon passes out through the excurrent siphon. How is this circulation explained? If a small piece of the gill of a clam or oyster is placed in a drop of the fluid found in the mantle cavity and examined under the compound microscope, the explanation is found. The surface of the gill is seen to be pierced by numerous holes. These holes,