434 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



They may be defined as a kind of water-moulds, wLich usually live 

 at the expense of dead and submerged animal and vegetable substances, 

 and are especially common upon dead insects and other invertebrate 

 animals. Their delicate hypliiie form a white cottony fringe to such 

 matters.* 



A dead fly which has fallen into water is a favorite uidus for Sapro- 

 legnia, the hyphte of which radiate from it in all directions, so that the 

 fly appears to be inclosed in a pale white fluff^^ ball. Careful examina- 

 tion shows that such a fly represents the soil in which an immense num- 

 ber of the minute Saprolegnuc are implanted. One-half of each fungus 

 consists of branching hyphiie, which answer, in a fashion, to the stem 

 and branches of an ordinary plant, and are visible externally; the other 

 half of the fungus corresponds, in the same general way, to the root and 

 rootlets, the hyphae ramifying in the interior of the fly, and the two 

 parts being connected by a portion which traverses the dense cuticle 

 with which a fly's body is coated. 



The stemhyphtie answer exactly in size and structure to the hyphas 

 of the salmon fnngus. Moreover, a large number of them terminate in 

 zoosporangiaof the same character, which evacuate their zoospores, and 

 are reproduced in the same way. 



Flies, or parts of flies, such as the legs, on which Saprolegnice are 

 healthily growing, can be isolated and watched for any needtul time 

 under the microscope, so that the whole process of the formation of the 

 zoosporangia and zoospores can betoUowed step by step. It may then 

 be observed that the simple sub-cylindrical free end of a hypha enlarges, 

 that protoplasm accumulates in it, and that its cavity finally becomes 

 shut off by a transverse partition from the rest of the hyi)ha, as a zoo- 

 sporangium, the summit of which is usually slightly conical. The pro- 

 toplasm is then seen to break up, simultaneously, into from eight or ten 

 to a hundred and fifty zoospores, according to the size of the zoosporau- 

 gium. The apex of the latter then opens and the zoosporongia are 

 emitted. Each zoospore, as it leaves the zoosporangium, is usually in 

 active motion, being propcdled by the rapid lashing of two vibratile cilia 

 which are attached to one point of its surface. After a few minutes it 

 becomes quiescent and surrounds itself with an extremely delicate trans- 

 parent coat. But this repose is of a very short duration, as it soon 

 emerges from its envelope, and moves about even more actively than 

 before. It has now an elongated oval shape, and has two ciba M'hich 

 proceed from one side of the oval. This second active state may last 

 for a day, or perhaps two; and it is obvious that, Irom the activity of 

 the motion of the zoospores, to say nothing of accidental currents, they 

 may thus be carried a long way from the parent stock. Sooner or later, 

 however, they again come to a state of rest, which is final, and they 

 then usually germinate. That is to say, one, or perhaps two, delicate 



* Whence the name aa/rpof, sajyros, rotten, and ?iiyvor, legnon, the edging of a gar- 

 ment. 



