DESCRIPTIONS OF TWO NEW LEECHES FROM PORTO RICO. 
215 
Annulation . — In the following description of the annulation, the annulus bearing the metameric 
sensible is regarded as the middle one of the complete somite (see Castle, 1900, and Moore, 1900). 
The description further serves to show how this view works out when applied to one of the live-ringed 
Hirudinidse. This leech possesses fifteen typical complete somites, being Nos. ix to xxm, inclusive 
(pi. 12, figs. 1, 2, and 3). The five rings constituting each are of approximately equal length, but, 
as indicated by the symbols on figures 2 and 3, they have very different morphological values, the 
middle annulus being equal to the first plus the second, and to the fourth plus the fifth. (See Moore, 
’98 and 1900. ) 
The structural details of the exterior of the complete somite xv are shown in figure 4, plate 12, 
the portion included between the lines dm and m representing the dorsal surface and between vm and m 
the ventral surface. Owing probably in part to the accidents of contraction, the annuli are of irregular 
length; they overlap somewhat at the posterior margin, and are in places marked by slight wrinkles, 
usually running transversely. More definite and constant longitudinal furrows, which tend to alter- 
nate in position on adjacent annuli, divide the surface into quadrate areas. According to their size, 
these areas bear one, two, or more of the conical sense organs each elevated on a papilla and surrounded 
by a group of small goblet-shaped organs and sometimes by smaller conical organs. When not too 
much displaced by contraction, these papilke form a range along the middle of each annulus. They 
vary much in size in different places and individuals, but are especially rough and prominent at the 
margins and posterior end of the body. 
The middle annulus, in addition to these non-metameric organs, bears seven pairs of very 
conspicuous metameric sensilhe which have the large size, the shape, and inclined position so well 
represented in Whitman’s (’86) figure of H. javanica. Typically four pairs of these are dorsal (pi. 12, 
fig. 4), the dorso-median ( md ), the dorso-lateral (di), the dorso-marginal (dma ) , and the supra- 
marginal (sm); three pairs are ventral, the submarginal ( sbm ), the ventro-marginal ( vma ), and the 
ventro-lateral (vl). They are of elliptical outline with a rather prominent axial ridge, along which a 
narrow white line (of transparent cells) runs. The dorso-median pair are broadly elliptical, situated 
close together with only one quadrate area intervening and inclined to the median plane at an anterior 
angle of about 30°. The dorso-lateral pair have the same form, but are of slightly larger size, being 
the largest of the segmental sensible. They are separated from the dorso-median by three or four 
quadrate areas, and lie almost exactly midway between the marginal line and the latter. They incline 
to the median plane at an angle of approximately 45°. The dorso-marginal are long and narrow and 
their inclination to the median plane approaches closely, or even reaches, 90°; they are commonly 
separated from the last described by three quadrate areas. Much smaller are the supramarginal 
organs, which vary much in shape, size, and position. 
The ventral sensilhe are all of approximately equal size and similar shape; the ventro-lateral have 
an inclination of about 35° and the others of about 90° to the median plane. The ventro-lateral and 
ventro-marginal lie closer together and nearer to the median line than the corresponding organs above. 
Of course, the angles of inclination vary and in any case can not be measured very accurately, so that the 
angular inclinations given are only approximate. The sensilhe situated at the margins of the body are 
peculiarly inconstant. Frequently one is represented by two or t hree smaller ones, or may become minute 
or altogether suppressed, or two may apparently unite into one, which occupies an intermediate position. 
Toward the anterior end the first incomplete somite met with is No. vm. Annuli m 2, 65, and 66 
are precisely as in the following somites. On the dorsal side 61 and 62 are still distinct, but much 
shorter, and the furrow which separates them has become faint. On the ventral side the furrow 
extends only a short distance mesiad from the margins, but the two rows of sense organs are apparent 
almost to the middle line, at which point the ring becomes entirely undivided, representing al. On 
somite vii the corresponding annulus exhibits a slight trace only of the furrow 61/62 on the middle of 
the dorsal surface, but two rows of -sense organs persist to the margins or even onto the ventral surface. 
Annuli 65 and 66 present exactly the same condition as do 61 and 62 of the succeeding somite (vm). 
vi is a typically triannulate somite above, but on the middle part of the ventral surface the furrow 
between ol and «2 has disappeared. The distinction between these two annuli is preserved across 
this space, however, by the persistence of two series of sense organs. On the dorsal side the presence 
of double series of sense organs on al and a3 suggests the growth potentiality of these rings also. On 
this somite the sensilhe of the dorso-lateral pair become modified as the last pair of eyes, which are 
the smallest of the series and have their axes directed outward and backward. Somite v is biannulate 
dorsally, with a faint partial furrow incompletely dividing the anterior annulus into two. Anterior to 
