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Hirudinei. 
Order II. 
Fig. 1. Hirudo medicinalis, the Leech, is the 
best known representative of this Order. It has a 
flattened body with short rings but no distinct head, 
rudimentary legs, nor bristles. It is reared in large 
numbers in Southern and Eastern Europe for medi- 
cinal purposes, and its body is capable of great ex- 
pansion when distended with blood. The sucking- 
Glass lite 
This Class includes a few marine genera -of 
comparatively slight importance, although zoologi- 
cally interesting in some respects. They have all a re- 
tractile proboscis, an unjointed body, and a vascular 
system. The sexes are separate, and the young 
undergo metamorphoses. Fig. n, represents a dark 
ereen Mediterranean species, Lonellia viridis. 
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mouth is armed with 80 small movable teeth, ar- 
ranged in a half-circle. The animals are herma- 
phrodite, and lay their eggs in capsules. 
Fig. m. Aulacostomum gulo, the Horse-Leech, 
is common in ponds and ditches. It is sometimes 
very annoying to horses and cattle, especially in 
Southern Europe and North Africa. 
Gephyrea. 
The proboscis is divided in front, and the expanded 
body bears 1wo hooked bristles at the extremity. 
The female is here figured; the male is wormlike, and 
extremely minute, and lives in the female like a 
parasite. The genus Szpunculus (S. Bernhardus is 
represented at fig. 0) may be recognised by the 
absence of the hooked bristles. 
Class IV. Nemathelminthia. 
These animals are interesting both from a 
Zoological and from a practical point of view. They 
may be recognised by their round thread-like or tube- 
like unsegmented bodies, which are destitute both 
of rudimentary legs and bristles. They have neither 
blood-vessels nor differentiated breathing-organs, but 
there is a nervous system, and sometimes organs of 
vision. They have special organs for offence and 
grasping, in the form of papilla, teeth, hooks &c., 
at the front extremity of the body, which show them 
Order I. 
The Thread-worms are characterised by their 
long cylindrical thread-like body. Some species are 
viviparous. Their development varies much. Some 
pass into an animal with the food, while others seek’ 
out their host themselves; and others again pass 
intervals of parasitism in alternation with a free life. 
Several species exhibit alternation of generations; 
i, e. the animal produces offspring wholly unlike the 
parents, while the latter in their turn reproduce the 
parent form, so that several generations are needful 
to complete a single life-cycle. 
One of the most famous of these worms is 
Trichina spiralis. The male is ‘1e¢ of an inch in 
length, and the female twice that size; and they in- 
habit the intestines of various mammals. Thus, a 
female living freely in the intestines of a pig, pro- 
duces from 1500 to 1800 living young, which im- 
mediately perforate the walls of the intestine, and 
fix themselves in the muscles, where they roll them- 
selves together, gradually invest themselves with a 
calcareous covering, and may remain thus for years 
(as at fig. p). They require another animal for their 
further development, and if they do not obtain it, 
they gradually perish, but if they are introduced into 
another animal with the flesh (as into a man) the 
calcareous shell dissolves and the animals develop 
and produce young which again bore through the 
integuments to fix themselves in the muscles of their 
new host. The injury which they thus occasion 
causes the painful and dangcrous discase called 


to be parasites on other animals. Most of them are 
such, either permanently, or at different stages of 
their life. The sexes are separate, and the young 
undergo metamorphoses, for these worms pass diffe- 
rent stages of their lives in different animals; and 
as it is necessary for them to pass from one ani- 
mal to another in order to attain their full develop- 
ment, its course often becomes very difficult to trace. 
They are divided into two Orders according to the 
form of the body. 
Nematoda. 
Trichinosis, but if they once fix themselves in the 
muscles without causing the death of their host, they 
are no longer inconvenient or dangerous, 
Fig. q. flaria medinensis, the Guinea Worm, 
is a native of the tropics. The male is unknown, 
but the female, which grows to the length of several 
feet, inhabits the muscles of man. The parasite is 
extracted by opening the sore, and very carefully 
winding out the animal on a stick, for if it breaks, the 
millions of embryos in the body of the mother set up 
dangerous inflammation. Its life-history is not yet per- 
fectly understood, but it is known that the young worms 
live in minute Crustacea (a species of Cyclops) and 
are probably swallowed with drinking-water. 
Gordius aquaticus is a small thread-worm which 
lives parasitically in insects, but also lives freely 
in water. Its history too is obscure. 
Oxyuris vermicularis is one of the commonest 
human parasites. The female attains a length of 
two-fifths of an inch, but the male does not grow 
to half that size. It is often found in the human 
intestines, especially in those of children. The eggs 
require no intermediary, but develop themselves at 
once into sexually mature animals, if they should 
again be introduced into the human body with food 
or drink. — Fig. r. Ascaris lumbricoides is another 
disagreable parasite which is found in the small in- 
testines of man Here an intermediary is probably 
needful to its development. The female is estimated 
to produce filty millions of eggs. 
