THE LEECHES OF JAPAN. 335 
many transverse rows as somites, and as many longitudinal 
rows as papille in a single ring. The longitudinal rows may 
be designated according to position, as median and lateral. 
The two median rows are the most prominent, and are placed 
somewhat nearer the lateral dark-brown stripes than the 
median stripe ; the two inner lateral rows are located just out- 
side the lateral stripes, and the two outer lateral rows just 
inside the marginal yellow stripes. Thus each of the three 
broad longitudinal areas of colour is marked by two rows of 
papille. 
In the aquatic Leech (figs. 10, 11, 18) we find six rows of 
spots which are plainly homologous with the segmental papille 
of the Land Leech, although smaller and only slightly raised 
into papilla-like protuberances. 
On the ventral side of both the land and the aquatic Leech 
are also found six rows of these segmental papille or spots ; 
but here they are so feebly developed that they might be over- 
looked. They are placed on the rings that bear the dorsal 
rows and are similarly disposed. 
These segmental spots have been described in various species 
of aquatic Leeches, but no one has hitherto studied their 
structure, or offered even a plausible suggestion as to their 
function. Their arrangement on the dorsal side, as shown in 
fig. 6, suggests an explanation of their nature, which is cor- 
roborated by a study of their histological structure. It is 
perfectly plain that the fifth pair of eyes occupy the places of 
two of these spots in the inner lateral rows. It is also easy to 
trace the median rows into the first pair of eyes. As will be 
shown more fully in describing the Medicinal Leech, the first 
pair of eyes must be genetically associated with the two 
median rows of segmental papillz; and all the remaining eyes 
with the two inner lateral rows. According to this view, the 
eyes and segmental papille were, primarily, morphological as 
well as physiological equivalents ; but this does not necessarily 
imply that they now have the same functional significance. 
The original segmental papillae may have represented sense- 
organs of a more or less indifferent order, among which, iu 
