THE LEECHES OF JAPAN. 405 
certainly the same, the time and conditions which brought the 
two sets of organs into existence cannot be identical in all 
respects. 
In respect to time of origin, the segmental sense-organs 
must be placed first, for the non-segmental organs are limited 
to a specialised part of the animal, and have undoubtedly 
arisen in response to the increased needs of this part. ‘There 
is not the slightest reason to suppose that they owe their 
origin to a multiplication of the segmental sense-organs by 
division. On the contrary, it seems quite certain that they 
must have arisen quite independently. 
Before considering the question of function, there are a few 
points of comparison to be noted between these organs and 
the lateral-line organ of the Fish. 
In the Teleostean Fish these organs pass through, in their 
early development, a stage which is identical with the simple 
epidermal thickening that remains permanent in Clepsine 
(see Fig. 2). In a somewhat later stage (Fig. 6) the periphe- 
ral cells develop hair-like extensions, which coalesce to form a 
Fic. 6.—Lateral-line organ of a Teleostean Fish at the time of hatching. 
ap. Strap-shaped appendage. ep. Epidermis. wz. Nerve. wz. s. Nervous 
stratum. 
delicate strap-shaped appendage. The addition of such a 
mechanism for raising the sensory power of the cells in a 
special direction does not of course, in view of the facts now 
well known in regard to the morphology of sense-organs, 
