164 MARINE INVERTEBRATES 
meaning “worm”; the prefixes Platy-, Nemat-, Troc-, meaning 
“ flat,” “thread,” “ wheel,” respectively ; while Annulata, meaning 
“yinged,” describes the segmented forms belonging to that 
phylum. 
There are vast numbers of parasitic worms, which live inter- 
nally or externally on their hosts, there being no animal of land 
or sea, of high or low degree, which is not subject to the affliction 
of these visitors. The parasitic worms are degenerate, some 
being without digestive organs, or without eyes, or without loco- 
motor organs, and so on, as the case may be, the host supply- 
ing the missing function. The life-history of these low forms 
is interesting; but parasitic worms do not come within the scope 
of this book, and are mentioned only to mark their place in the 
series. 
PHYLUM PLATYHELMINTHES 
FLATWORMS 
The flatworms have a flattened body, more or less compressed 
in the different classes. Ordinarily the body is very thin, and, 
when short, has a leaf-like form; when long it is ribbon-like. 
Some species are thick in the middle and thin at the edges. They 
have a dorsal, or upper, and a ventral, or under, surface; a right 
and a left side; an anterior and a posterior end. The anterior 
end is carried forward and has some of the characteristics of a 
head, though a distinct head is not apparent. The mouth is on 
the ventral surface. In some species it is in the middle of the 
iength of the body; in others it is before or behind this point. 
The flatworms are the first. animals to assume pronounced 
bilateral symmetry. They are soft-bodied, having no supporting 
skeleton, and they have no segments, or divisions, such as are 
found in higher types, asin Annulata. They have no body-eavity, 
the space between the organs and the body-wall being filled with 
tissue. The alimentary canal has no anal aperture, the excretions 
being carried off by a water-vascular system consisting of branch- 
ing vessels which end in minute bundles of vibrating cilia, called 
ciliary flames. These flames communicate with the exterior 
through small pores or flame-cells. Their sense-organs are eyes 
