14 HIRUDO. 
I was afterwards gratified to find that it had come under the notice 
of Dr George Johnston, who, considering it a leech, had named it 
Udonella caligorum. 
This is a minute white cylindrical animal, with a sucker at the 
posterior extremity, whereon it rises erect, or swings the body in all dif- 
ferent positions. The integument is too opaque to expose the contents 
distinctly. None of my specimens extended to more than two lines, those 
of Dr Johnston have equalled four, nor have they been alike opaque. 
This animal is not always present on the Caligus, but it sometimes 
appears in great numbers. By gross enumeration about 150 were com- 
puted as adhering to a dead Caligus. 
There are frequently present along with specimens on the Caligus 
numerous clusters of extremely minute capsules, as they appear ; some 
full, and others of the same cluster empty. But whether these belong 
to the Udonella is uncertain. 
Extremely minute specimens of the Udonella are occasionally seen 
on the neck of the Hirudo muricata, lurking among the prominences. 
Though a longer series of observations, conducted under favourable 
circumstances, might be requisite to determine the precise position of the 
Udonella caligorum, it may safely receive a provisional place among the 
Hirudines. 
§ 6. Hirupo Ocro-ocuLata, seu vuLGARIS.—The Kight-eyed Leech.— 
Plate II. 
I have not ascertained that more than four marine leeches, including 
the HZ. grossa and Udonella, belong to Scotland. But those inhabiting the 
fresh-water are perhaps more than in due proportion to the space which 
the whole territory occupies on the surface of the globe. If the number 
specified here have fallen under the notice of one individual, it may be 
readily presumed that more have occurred to others. I consider it for- 
tunate when we can study the genera of animals without having re- 
course to those of foreign seas. Ifthe matter be viewed philosophically, 
