SEGMENTED WORMS AS PARASITES 199 



the neighbourhood of the tail, and root - like threads grow out 

 from it in all directions. Fed by these it grows rapidly, and 

 exerts so much pressure on the muscles and skin which are 

 placed between it and the under side of the tail that they be- 

 come thinner and thinner. Ultimately, as the final result of this 

 process, the parasite projects to the exterior, its roots, however, 

 remaining inside the crab. 



Some of the higher Crustaceans belonging to the group of 

 Slaters (Isopoda) are also parasitic, and have undergone profound 

 modifications. 



SEGMENTED WORMS (ANNELIDA) AS PARASITES 



We are here concerned with various Bristle- Worms (Chaeto- 

 poda) and Leeches (Discophora). 



BRISTLE- WORMS (CH^ETOPODA) AS PARASITES. A number of 

 marine worms are external parasites upon hosts of widely different 

 nature, including star-fishes, sea-urchins, 

 sea - cucumbers, corals, and even other 

 annelids. Cases have also been described 

 where one species of marine worm lives 

 parasitically within the body of another 

 species. Much more interesting than 

 these, however, are certain small flattened 

 creatures (e.g. Myzostoma, fig. 1147), which 

 live upon, or more rarely within, feather- 

 stars and sea - lilies, sometimes causing 

 gall-like growths that serve as habitations. 

 Since these curious little parasites possess Fi s- "4?- under side of Myzostoma, 



L enlarged, s, Suckers; f t foot-stumps. 



a small number of foot-stumps, each ter- 

 minating in a pair of bristles, they are probably to be regarded 

 as bristle-worms that have become modified in consequence of 

 their mode of life. And this view is supported by the fact that 

 they begin existence as larvae which closely resemble those of 

 typical worms of the kind. 



One large group of Annelids (Few-bristled Worms, Oligochata\ 

 of which earth-worms and certain freshwater forms are the typical 

 representatives, includes a small number of species resembling 

 leeches in appearance, as well as in the fact that bristles are 

 entirely absent. They are ectoparasites upon crustaceans, the 



