332 Cc, O. WHITMAN. 
With respect to the three somites which follow the three 
cephalic somites, it should be noticed that, although the number 
of rings in each is the same, they do not exhibit the same 
degree of concentration and development. The 4th somite 
is represented on the ventral side by the coalesced buccals and 
the 6th ring ; and on the dorsal side the first buccal (4th ring) 
is composed of fewer areas than the succeeding rings. The 
difference between the 5th and 6th somite is slight on the 
ventral side, but well marked on the dorsal side by the presence 
of the fifth pair of eyes in the former. Thus the head and 
anterior end of the body of the Land Leech, especially in com- 
parison with the corresponding portions of the Medicinal Leech, 
plainly illustrate an order of events which may be called the 
law of centripetal abbreviation; and at the same time they 
show a strict correlation between the grade of development 
and specialization and the degree of abbreviation. 
Remembering that the 4th ring of the Medicinal Leech 
is wanting in the Land Leech, it becomes very easy to identify 
the rings of the latter with those of the former, and to see 
that the sexual orifices are situated between homologous rings 
in the two cases. 
Comparing now the hind end of the body of the Land Leech 
with that of the Medicinal Leech, we find that the direction of 
abbreviation is here also centripetal. In fig. 10 we find twenty- 
six somites, of which six anterior and four posterior are abbre- 
viated ; while in the Land Leech there are, apparently, only 
twenty-three somites, of which six anterior and one posterior 
are abbreviated. In both cases then there are six abridged 
somites followed by sixteen unabridged; and this leaves only 
four rings in the Land Leech to offset nine in the Medicinal 
Leech. There can be but little doubt that the first twenty-three 
somites correspond in the two species ; and this being assumed, 
we may inquire how far the remaining posterior rings can be 
identified. The four posterior rings of the Land Leech (fig. 7) 
appear at first sight to represent a single somite ; but this view 
is rendered doubtful by the fact that no somite of four rings 
occurs in Hirudo. An examination of a large number of Land 
