THE LEECHES. 241 



for no profession is so self-denying or so generous as the medical. The employment of Leeches 

 is diminishing year hy year, and now they are hardly ever used in England. In Paris, between 

 the years 1825 and 1830, three millions of Leeches were used, and it was calculated that thirty 

 millions were employed in France and England every year. All the ponds and marshes where 

 Leeches were bred, or were found in a state of nature, were nearly exhausted. Now the rate of 

 mortality is less, and Leeches seek other prey. 



The Leeches have ocelli in the form of black specks ; they vary in number according to the 

 genus, and are placed in pairs. There may be from one to five pairs, and they are very sensitive, 

 and are disposed on the anterior part of the front sucker. The Leeches appear to dislike certain scents 

 and greasy substances, and their skin is exceedingly sensitive to pungent substances, such as salt. 

 Moreover, from their behaviour in rising and sinking in 

 clean water at certain times, they would appear to feel 

 alterations in barometric pressure. 



With regard to their bodies, the numerous rings do 

 not correspond to as many segments, there being from 

 three to five to each somite or segment. The skin is 

 smooth and rarely tuberculated, and it has two kinds of 

 unicellular glands. One set secretes mucus generally, or only 

 at the mouth or sucker, and the other produces a chitinous MEDICINAL LEECH. 



material which forms cocoons, in which the eggs are included. 



Three layers of muscular fibres exist the circular, the radiating, ana the longitudinal. The nervous 

 system consists of a ventral cord, divided into ganglia at regular intervals, and there are twenty- 

 three in the common Leech, and the anterior and posterior ganglia are the largest, and seven are fused 

 in front, into one mass. Above the pharynx the branches of the front ventral ^cuiglion unite to, form 

 a ring and an upper lobate pharyngeal ganglion. A single nerve lies beneath the intestine, and it 

 has ganglion cells ; and many nerve-twigs terminate in the centres of little depressions covered by 

 clear cells the cup-like organs which are situated on the head and hinder, but not hindmost rings of 

 the skin. The mouth is in or below the anterior sucker in the sub-class as a whole, it leads to a 

 muscular pharynx, and some genera have a protusible proboscis which has retractor imiscles, but no 

 teeth. The jaws are made up of calcified cbitine. There are salivary glands in the gullet, and the 

 oesophagus leads to a long stomach, which has nine side-pouches or caeca. These cseca open 

 into the stomach, and vary in number with the genera ; usually they are simple, but in Clepsine 

 they are branched. The pylorus has a circular contracting muscle, and the short intestine 

 passes backwards between the two hindmost caBca, and the anus is dorsal above the hinder sucker. 

 The circulatory system contains a red fluid with colourless corpuscles, and it flows in a small body easily 

 between the organs and the skin, which resembles a series of sinuses or narrow ways. These may 

 form two lateral pulsating vessels. In. the Leech this " pseudo-hsemal " system consists of a median 

 dorsal vessel, a median ventral cavity, in which the ganglionic nerve-cord lies, and two longitudinal 

 trunks which anastomose with one another, and give off a network of vessels to the muscular layers. 

 The respiration is effected by the skin, and in the genera Branchelion and Ozobranchus there are some 

 processes of the skin at the margin of the body, which may have to do with respiration. The 

 excretoiy organs, are tubes with glandular walls symmetrically arranged along the ventral aspect ; 

 they are either closed internally or open within, by a ciliated funnel-shaped orifice, while the outer 

 opening may be on a small wart or tubercle on the side of the body. These are called segmental 

 organs. 



One great Leech belonging to the genus Macrobdella from "Valdivia is an internal parasite, and 

 measures two feet five inches in length. In a genus which frequents shell-fish for many species live 

 in the sea the skin is ciliated. In another there are tubercles, and one genus has bristles. All the 

 Leeches lay eggs, and they may be deposited singly or in numbers, and in this case they may be 

 covered with a viscous web or with a spongy envelope called the cocoon, as already mentioned. When 

 the young are hatched, they keep within the cocoon, and in from twenty-one to thirty days burst 

 forth, and either keep close to their envelope or their mother, for a short time afterwards. They 

 do not undergo metamorphosis, and whilst in some the sexes are separate, they are united as a rule. 

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