NEUEIS 



189 



worm known as the " blood-spot " (Polycirrus, Fig. 182) is 

 retluced in size, and instead of gill tufts it has a large number of 



Fig. ISl. — Amphitnto, removed from it.s Fig. 182. — Polj'oirrus, the blood 

 tube. Nat. size. Photo, by W. H. C. P. spot. Nat. size. Photo. 



thread-like organs, the cirri, at the head end. Certain species, 

 such as Cistenides (Fig. 183), form complicated tubes out of 

 grains of sand cemented together in a regular arrangement so 

 as to form a firm wall. 



Finallj', Serpula ^ secretes crooked, round, calcareous tubes, 

 which may be found adhering to stones near low water 



^ From serpens, serpent. 



