26 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
largest in the early spring immediately before the eggs are laid, when they 
often extend the whole length of the genital region and are looped or folded, 
as are the seminal vesicles ; they are smallest immediately after the egg-laying. 
A mean between these two extreme conditions is shown in Figure 4. 
The time of egg-laying, as well as of spermatophore formation, depends upon 
the warming of the water in the spring. One can hasten both processes by 
bringing the animals for a few days into a heated room. Around Cambridge 
the eggs are laid mostly in the months of April and May. Small-sized indi- 
viduals, however, may come to maturity later in the season, even as late as 
September. 
The eggs are pink in color and about 0.3 mm. in diameter. They are 
attached to the under surface of the body in groups of two to eight eggs each. 
Each group is enclosed in a separate, delicate, transparent sac, which adheres 
to the under surface of the body. The sacs are arranged in two longitudinal 
rows close together, one on either side of the median plane of the body. The 
more posteriorly placed sacs usually contain more eggs than those farther 
forward. 
I have not observed the process of egg-laying, but believe that the eggs of a 
single sac are laid at about the same time, that they are then crowded back as 
far as possible under the body, and that there is poured over them a secretion 
from the clitellar glands which hardens into the delicate wali of the sac. After 
a period of rest, during which the body is closely applied to the group of eggs 
so that its sac becomes fastened to the body, another group of eggs is laid, and 
so on until all the mature eggs have been expelled from the ovary. The cli- 
tellar glands are deep-seated, unicellular epidermal glands opening on the 
ventral surface in the vicinity of the female genital pore. They can be 
demonstrated by methylen-blue staining. 
Animals which are kept in aquaria lay their eggs at night, and always com- 
plete the process in a single night, so that all the eggs borne by an individual 
are in about the same stage of development at one time. 
I think it probable that the egg sacs are arranged in the order laid, from 
behind forward, for in one of the most anterior sacs a single egg is occasionally 
found, but never in one of the more posterior sacs have I abeceed so small a 
number. The number of eggs laid by an individual depends. upon its size. 
An animal thirteen mm. long (when fully extended) was found to have laid 
sixteen eggs; another twenty-six mm. long was found carrying forty-five eggs. 
The average number for nine individuals examined at one time was thirty-one. 
The usual number of egg sacs formed is six or eight; in one case examined 
it was ten. The average number of eggs found in a sac is about four ; for the 
most anterior pair of sacs it is three. 
e. DiGESTIVE- TRACT. 
The position of the mouth (or, Figures 3, 7), except when the body is much 
contracted, is anterior to the eyes, in the third somite (ring 2). 
It leads dorsally into the pharyngeal sac (sac. phy., Figure 7), which con- 
