226 ZOOLOGY. 



The student, in familiarizing himself with the structure 

 and mode of growth of the leech, the common earth-worm 



Fig. 148. Transverse section of a vrarm, of AmpMoxus, and a higher vertebrate 

 contrasted. , skin ; 6, dermal connective layer; c, muscles; d, segmental organ ; h, 

 arterial, and i, venous blood-vessel ; g, intestine ; I, 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. 149), 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. 150) 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 tubes opening at one end at regular intervals on the 

 under side of the body, and ending in a non-ciliated coil 

 (Fig. 149, r) in. the leech, or in the smaller fish-leech, Clep- 

 sine, open into the venous sinus by ciliated, open mouths. 



