CONDITIONS OF LIFE 303 



Fierasfer is a fish of slender shape, from four to seven inches in 

 length ; its whole body is very transparent with only traces of 

 pigment in the skin ; it has pectoral fins, but no pelvic, the 

 dorsal fin extends from a little behind the head to the posterior 

 extremity, which is tapering and pointed, the anus is near the 

 head and the ventral fin extends from it to the end of the body ; 



Fig. 24. — Fierasfer and Holothurians (after Emery). 



there is no tail fin. In an aquarium the fish is observed to 

 seek the anus of the Holothurian with its head and then to 

 bend round the extremity of its tail and insert it into the aper- 

 ture after which it straightens itself and moves backwards until 

 it has passed completely into the body of its host. Within the 

 latter it is found to be contained within the cavity of one of the 

 tubes which open into the cloaca, and which are known as the 



