164 MARINE INVERTEBRATES 



meaning "worm"; the prefixes Platy-, Nemat-, Troc-, meaning 

 " flat," " thread," " wheel," respectively ; while Annulata, meaning 

 "ringed," 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 



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 

 length 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, as in Annulata. They have no body-cavity, 

 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 extei'ior 

 through small pores or flame-cells. Their sense-organs are eyes 



