The Annelides include the highest and most specialized Worms. They have many segments, spines, or suckers for locomotion, a supra-esophageal brain, a ventral chain of ganglia, and usually a closed blood sys

tem.

There are two principal classes: Chatopoda, or bristlefooted worms; and Hirudinea, or leeches. The former

class includes the earthworms (Lumbricus and Allolobophora), the sandworm (Nereis), and the lobworm (Arenicola).

The earthworm develops from eggs laid, several in a capsule, in the earth or near refuse heaps, under boards

and straw. Its cylindrical body consists of numerous segments, the wall being very muscular and covered by a tough, smooth, transparent cuticle. The body cavity is subdivided by numerous transverse membranous partitions. The digestive system extends throughout the body and there are well-developed nervous and

ph.

h

r

dv

-W

FIG. 52. -
ternal anatomy of the
anterior region. The body

Earthworm, in

has been opened along the

circulatory systems. The former lies mainly below the digestive organs and consists of a nerve cord running the length of the body. This cord is composed of pairs of ganglia connected by longitudinal and transverse. branches. Above the mouth opening is a pair of ganglia forming the "" 'brain." The larger blood vessels surround the esophagus and one, the dorsal vein, lies above the digestive system and may be seen through the skin on the back. There are no welldefined sense organs, although, judging from experiments, the earthworm is sensitive to touch, is affected by "heart";, muscular changes in the intensity of light, and gives evidence of having the sense of taste. Respiration is carried on by the vascular skin, there being no lungs or gills. Earthworms feed subneural blood vessel. upon decaying vegetable matter and upon organic particles contained in the earth, swallowed in the process of making the burrow, or for the sake of the contained food. The refuse from the body is piled up around the mouth of the burrow in the form of pellets. The amount of earth annually brought up from the deeper layers of the soil is sufficient to be of considerable geological and economic importance.

dorsal line and the inter

nal organs turned to the

left; ph, pharynx; h,

partitions; w, wall of body; r, reproductive or

gans (in part); du, dorsal

blood vessel; ¿, intestine;

g, "brain"; vn, ventral

nerve cord; si, subintestinal blood vessel; sn,

The earthworm belongs to the subclass Oligochata, the members of which have but few bristles on each segment.

Nereis lives in the sea, under rocks and among seaweeds. Like the earthworm, it has a distinctly segmented body. There is a well defined head, bearing sense organs, as eyes and tentacles. The throat is provided with two protrusible jaws, by means of which the worm seizes its food, often living prey (Fig. 215). Each segment bears a pair of flattened, paddlelike parapodia, which enable the worm to swim rapidly. The arrangement of the digestive, nervous, and circulatory systems is much like that seen in the earthworm.

Nereis is a member of the subclass Polychata, which is characterized by the presence of numerous bristles on each segment.

The leeches are externally segmented, usually flattened, and have a sucking disk at each end of the body. The mouth is in the anterior disk and is provided with three semi-circular, saw-toothed jaws, by means of which the leech makes the incision through which it sucks the blood of its prey. The disks are also used for locomotion. The digestive system is very capacious, and some leeches can live even if not fed more often than once in two or three months. Leeches are generally freshwater animals, though some kinds are found in the sea and others live on land.

This is larger than all the other branches put together, as it includes the animals with jointed legs, such as crabs and insects. These differ widely from the molluscan type in having numerous segments, and in showing a repetition of similar parts; and from the worms

DODGE'S GEN. ZOÖL.-7

in having jointed appendages and a definite number of segments.

The skeleton is outside, and consists of articulated segments or rings. The limbs, when present, are likewise jointed and hollow. The jaws move from side to side. The nervous system consists mainly of a double chain of ganglia, running along the ventral surface of

the body under the alimentary

canal. The brain is connected to the ventral

ganglia by a ring encircling the gullet. The alimentary

canal and the circulatory apparatus are nearly straight tubes lying lengthwise—the one through the center, and the other along the back. The skeleton is composed of a horny substance (chitin), or of this substance with carbonate of lime. All the muscles are nearly always striated.

There are five classes, of which the first almost exclusively is water breathing, having gills, and the others principally air breathing, being provided with trachea.

17

The Crustacea, with few exceptions, are water breathing Arthropoda, usually with two pairs of antennæ. Among them are the largest, strongest, and most voracious of the branch, armed with powerful claws and a hard cuirass, bristling with spines. Although constructed on a common type, crustaceans exhibit a wonderful diversity of external form: contrast, for example, a barnacle and a crab. We will select the lobster as illustrative of the entire group.