176 



Annelida. 



Ivd 



\o 



finally unite and open by an unpaired aperture in front of the female 

 pore. The eggs are laid in chitinous capsules (cocoons), usually 

 several together, with a eertain amount of 

 albumen. The capsules, which are formed by a 

 hardened secretion of the skin-glands, have either 

 a smooth surface, or are, as in the Medicinal 

 Leech, covered with a s]iongy case (hardened 

 frothy mucus). The young ones leave the cocoon 

 in the form of the adult. 



Each egg is, of course, covered by an egg-meiii)iraiio : 

 the emljryo of the Gnathobdellidw, whore the eg^'s are 

 very small, soon bursts this covering, and lies free in the 

 albumen, upon which it feeds, and thus gTows rajjidly. 

 It is in this stage very different from its later forms, iind 

 possesses several provisional organs (pharynx, muscles, 

 etc.), which atrophy, and are replaced by the permanent 

 organs before it leaves the cocoon. The Gnathobdellidtc 

 may therefore be said to undergo a metamorphosis 

 within the cocoon. In the Rhyncho) iddlida), whose 

 eggs are larger, this does not occur. 



The Leeches, which, comjiared with the Ohixjio- 

 poda, form a small group, are relatively well reju'c"- 

 seuted in fresh water ; still a considerable number 

 are marine. Some are terrestrial (in the tropics), 

 others frequently go on shore. They are predatory, 

 or are temporary parasites, sucking the blood of 

 larger animals ; some are stationary parasitc\s. They 

 cree]:> about in the well-known manner by mea.us 

 of their suckers, but are also able to swim by 

 serpentine movements of the body. 



1. GnaihohdelUdiB. With jaws. Anterior adhesive 

 organ divided into an upper and a lower hp. Eggs 

 small; the young ones undergo a kind of metamorphosis within tho cocoon. 

 All fresh-water or teiTeatrial. 



(a) The Medicinal Leech (Hirudo medicinalis), a fresh- water form, 

 varying in colour, occurring in different pai-ts of Eui-ope, and in England. Its 

 jaws are very strong and have pointed teeth. The dorsal surface is a grc'euish- 

 grey, with reddish longitudinal stripes, flecked with black : the ventral surface is 

 paler, but speckled. Ten eyes. The spongy egg-capsules arc laid on land, inbiiiiks. 

 To this genus belongs the well-known East Indian L a n d-1 e o c h (H. ceyloniea). 

 Hsamopis vorax is allied to the Medicinal-leech, which it resembles in shape and 

 size. It is indigenous to S. Em-ope and N. Africa. It frequently enters tlio 

 nostrils, pharynx, and throat of different Mammals with drinking-water, and may 

 occasion serious inconvenience. 



(6) The Horse-leech (Aulastomimi gulo). Very common in fresh watci- 

 in England, of a similar size to the Medicinal-leech. It is fi-pqnently mistaken 

 for Hcemopis vorax. The jaws are less developed than in the Mndicinal-leech. It 

 attacks no Mammal, but lives on Earthworms and small aquatic animals. It is 

 gi-eenish-black above and yellowish-bi-own below. Ten eyes. The egg-capsules 

 are like those of the Medicinal-leech, and are laid on land. Species of the genus 

 Nephelis are also frequently met with in fresh water ; they are shorter and 



Fig. 138. Genital 

 apparatus of a Leech. 

 n ventral nerve cord, 

 ovary, u oviduct, 

 t testis, vd vas defe- 

 rens, vs coiled part of 

 vd, g glands, p penis. 

 — After Spengel. 



