ZOOSPORES. 223 



spores absorb their cilia, and surround themselves with a 

 gelatinous envelope, when each breaks up into sixteen cells 

 (zoogonidia) and gives rise to a new colony {L and M, Fig. 

 149). 



Pandorina is nearly related to Volvox (see p. 243), from 

 which it seems a violence to separate it. It occurs in pools 

 of fresh water (in Europe) as minute green spherical ccenobia, 

 3 mm. (.012 inch) in diameter. 



295. — Hydrodictyon, the Water Net, is a common plant 

 in ponds and sluggish streams. It is, when full grown, a 

 tubular net, composed of a multitude of elongated cells, 

 which are attached only at their ends ; the net sometimes 

 attains a length of 25 to 30 centimetres (10 to 13 inches), 

 and the cells which compose the meshes are in such speci- 

 mens 7 to 8 mm. (^ inch) long. The 

 reproduction is as follows : The pro- 

 toplasmic contents of certain cells 

 break up into a large number of 

 daughter - cells (macrozoogonidia), 

 there being often as many as 7000 to pjg iso.-Part of a ceii of 

 20,000: these soon arrange them- Syfodicti,onvtricuiatmi,m 



' ' o which the macrozoogoniclia 



selves within the mother-cell so as are bcgiimingto arrange them- 



. . selves so as to form a minia- 



to form a miniature net (Fig. 150), ture net within tue mother- 



1 J.- f cell.-After CErsted. 



which IS freed by the absorption of 



the walls of the mother-cell. Under favorable conditions 

 the young net attains full size within a month. A second 

 mode of reproduction is known, or partly known. In cer- 

 tain cells, in the division of their protoplasmic contents, in- 

 stead of giving rise to the comparatively large macrozoogo- 

 nidia, they produce an extremely large number (30,000 to 

 100,000) of very small ciliated swarm-spores (zoospores, or 

 the chronizoospores of Pringsheim), which, after s^vimming 

 about for a time, acquire thick walls, and fall to the bottom 

 of the water, where they remain in a resting state. Upon 

 their germination they pass through a number of curious 

 stages, and finally give rise to small nets. Suppanetz is said 

 to have witnessed the conjugation of the swarm-spores within 

 the mother-cell, or immediately after their emission.* 



* Qr. Jour. M.c. Science, 18V5, p. 399. 



