to 
ZOOLOGY 
for example, the segmentally-arranged nerve ganglia, may fuse 
together, and thus the segmentation becomes irregular, 
The Hirudinea are the first group in which segmentation 
forms a distinctive feature. The integument is ringed, and in 
the medicinal leech, Hirudo medicinalis, five annuli correspond 
with a segment, in Pontobdella four, and in Lranchellion three. 
The limits of each true segment are, however, marked out by 
the arrangement of the colour bands. 
A cuticle corresponding with the mucous tubes of Nemer- 
tines is formed from the secretion of unicellular glands; it is 
constantly being worn off and replaced. 
The body-cavity is much reduced by the great developement 
of muscles and connective tissue, and in the medicinal leech 
its chief remains form the dorsal and ventral sinuses, it is, 
however, more conspicuous in the Rhynchobdellidae. 
The bodies of those forms, such as Clepsine and Nephelis, 
where the muscles are strongly developed and the connective 
tissue is sparse, are peculiarly firm and rigid, but forms lke 
Aulostoma, and to a less extent Hirudo, where the connective 
tissue predominates, are extremely limp and flabby. The cells 
of this connective tissue are embedded in a gelatinous matrix, 
and they may assume the following characters : (1.) fat cells, or 
cells crowded with fat globules, common in Clepsine; (ii.) 
elongated branched cells crowded with globules which are not 
fat, these pass into fibres at times; (ill.) pigment cells; (iv.) 
vaso-fibrous and botryoidal cells, forming a tissue which 
is composed of certain rounded cells crowded with brown pig- 
ment and arranged in rows; by a change in their interior, 
channels arise which pierce the cells, and these ultimately open 
on the one side into the closed system of blood-vessels, and on 
the other into the coelomic sinuses. The botryoidal tissue 
appears to pass into the vaso-fibrous tissue by the cells 
dividing and becoming small, and the walls becoming very 
thin, and the nuclei of the cells dropping out. Many of the 
minute capillaries thus formed run between the columnar 
epithelial cells of the ectoderm, and here the oxygenation of 
the blood takes place. 
In Nemertines the vascular system is one with the 
coelomic. The space which contained the corpusculated fluid 
