THE WORMS 63 
covered with cilia, create the currents of water which 
bring minute organisms serving as food within reach of 
the mouth. 
65. Development.—Unlike the earthworms, the Poly- 
cheetes lay their eggs in the sea water, where they are left 
alone to develop as best they may. Both the male and 
female Nereis, as the egg-laying time approaches, undergo 
remarkable changes in their external appearance, resulting 
in the form shown in Fig. 36, A. 
They are now active swimmers, and 
thus are able to scatter the fertilized 
eggs over wide and more or less favor- 
able areas. The young also for a 
time are free-swimming, but finally 
end their migrations by settling to 
the sea bottom, where they gradually 
attain the adult condition. 
As in some of the flatworms, re- 
production may also arise asexually 
by the division of the animal into two 
or more parts, each of which subse- 
quently becomes a complete indi- 
vidual. In other species growth of 
various parts may result in two com- 
plete worms at the time of separation ; 
and from such forms we may trace a ag 
fairly complete series up to those in \<¥Y 
which the original parent breaks up Fs. 40.—A leech (Macrobdel- 
: 3 7a). Right-hand figure 11- 
into twenty to thir ty young. lustrates alimentary canal. 
66. The leeches.— At first sight ph, pharynx; ¢, crop; p, 
the leeches (Fig. 40), or at least the — {2teral Pouches ssf im 
smaller, more leaf-like forms, might 
be mistaken for flatworms, especially for some of the para- 
sitic species. As in the latter, the mouth is surrounded by 
a sucker, and another is located at the hinder end of the 
body, but beyond this point the resemblance ceases. The 
