164 ZOOLOGY. 
The student, in familiarizing himself with the structure 
and mode of growth of the leech, the common earth-worm 
Fig. 112.—Transverse section of a worm, of Amphioxus, and a higher vertebrate: 
contrasted. @, skin; 6, dermal connective layer; c, muscles; d, segmental organ ; h, 
arterial, and 2, venous blood-vessel ; g, intestine ; Z notochord.—After Haeckel. 
and the Nereis, will obtain a good idea of the essential char- 
acteristics of the entire class. 
Order 1. Hirudinea.—In the leech (Fig. 113), Hirudo 
medicinalis Linn., the type of the first and lower order, the 
body is somewhat flattened and divided into numerous short, 
indistinctly marked segments, not bearing any bristles or 
appendages. The head is small, with no appendages, bear- 
ing five pairs of simple eyes, while each end of the body ter- 
minates in a sucker. The mouth is armed internally with 
three pharyngeal teeth arranged in a triradial manner, so 
that the wound made in the flesh of persons to whom the 
leech is applied consists of three short, deep gashes radiating 
from a common centre. The stomach (Fig. 114) is large, 
with large lateral diverticula or lobes, while the intestine is. 
small. The nervous system consists of a “‘ brain” and ven- 
tral ganglionated cord. 
The vascular system is complicated, consisting of a median 
dorsal and a ventral vessel, and two lateral vessels ; all these: 
anastomose or interbranch, and the blood which courses. 
through them is red, but is said to contain no corpuscles. 
The segmental organs, so characteristic of the Annulata, 
are well developed in the leech, consisting of about seventeen 
pairs of tabes opening at one end at regular intervals on the 
under side of the body, and ending in a non-ciliated coil. 
(Fig. 113, r) in the leech, or inthe smaller fish-leech, Olep- 
sine, open into the venous sinus by ciliated, open mouths. 
