148 FISH ENEMIES 



Mites (Hydrachna). These odd-looking little balls of intense red 

 are sometimes placed in small tropical aquaria with fishes. This is in- 



Fic. 146. Water Mite (Enlarged about four times) 



advisable, as they are parasitic. It is doubtful whether they actually 

 kill their host. They are common in the still water of lily ponds. 



Hydra. Although Hydra is not an insect, we include it here as 

 an important enemy of young fishes. Hydra is a polyp which attaches 

 itself to plants, stones or the sides of the aquarium. Being thread- 

 like in appearance, it is apt to be overlooked, especially by the inex- 

 perienced. It is usually introduced with living food caught from 



Fig. 147. Hydra (Greatly enlarged) 



pools. The spores are so small as to be unrecognizable, and there- 

 fore they cannot be avoided. The animal itself has a cylindrical 

 body with from 5 to 12 tentacles surrounding a mouth. The shape 

 varies so amazingly that an accurate description is difficult. The two 

 figures shown are of the same individual. When alarmed the tentacles 

 are entirely withdrawn, making recognition still more difficult. The 

 figure at the left is the usual form when not alarmed. In this condi- 

 tion it reaches from ^/^ to ^4 inch in length, but the body may be con- 

 siderably thinner if food is scarce. 



Propagation is by budding, by spores and by division. If one of 

 the tentacles be cut oft' it will develop into a perfect hydra. 



They rapidly deplete an aquarium of daphnia and newly hatched 

 fishes. When a school of fry under 5-16 inch long is disappearing 

 without apparent cause, it will be well to take a sharp look for some 

 harmless-looking hydra. 



