ZOOLOGY. 323 



to the ground, where they remain quietly for two or three 

 weeks, gradually swelling and changing form. "At this 

 time the pupa state is assumed, but not by shedding any 

 skin, as do true insects in undergoing their transformations. 

 New legs, feelers, and mouth-parts form under the old skin, 

 which, with its now useless legs, distends so as barely to 

 cover the new parts, which are all appressed to the body, 

 very much as in the pupa of a beetle." Finally, both the 

 distended larval skin and the new one that incases the pupa 

 burst, and release the mite in its eio-ht-leo-o-ed adult form. 

 It appears that from the time this mite hatches, through all 

 its growth and changes, but one molt takes place. The 

 adult mite passes the winter in the ground, and is active 

 whenever the temperature is a few degrees above freezing- 

 point. Professor Riley has also reared Trombidium mmca- 

 rum Riley from the larva which lives on the common house- 

 fly. He also figures and describes the transformations of 

 JSydrachna belostomw Riley, an aquatic mite which usually 

 infests the large water-bug (Zaitha) ; as many as 500 some- 

 times occurring on a single bug. They fasten themselves 

 and penetrate the chitinous skin of their host with their 

 maxilla 1 , which form a long, pointed thread. The body be- 

 comes sac-like, and transforms into the pupa state within 

 this sac, which finally bursts to release the adult mite. This 

 bag-like larva was looked upon as an egg by many old au- 

 thors, and was made the type of the genus Achlysia by Au- 

 douin. Mr. Riley's article is printed in advance from tho 

 Rejwrt of the United States Entomological Commission. 



The development of the crayfish has been freshly studied 

 by Reichenbach, w T ho supplements the works of Rathke, Le- 

 reboullet, and Bobretsky. He has found that many of the 

 endodermal cells of the ordinary columnar form are lobed at 

 the end towards the yolk, and give off more or less fine 

 threads of protoplasm, which pass between, and in some 

 cases surround, the yolk spheres. These cells evidently ab- 

 sorb the nutritive matter of the yolk, "not by a passive 

 process of diffusion, but by an active process of ingestion, 

 the food particles being immediately ' plunged into the liv- 

 ing protoplasm of the cell,' and there digested." This active 

 swallowing of particles of the yolk by embryonic cells was 

 first observed by Lankester in tho egg of the cuttle-fish. 



