HYDROGASTRUM. 



[ 332 ] 



HYMENIUM. 



a homogeneous mass, and the rudiment of the 

 first starch-granule soon appears, while the 

 gonidia grow out into a tubular form, acquire 

 a cellulose membrane, and collectively form 

 a new net, which becomes free by the total 

 solution of the parent-cell. These gonidia 

 appear to possess four short cilia ; their mo- 

 tion lasts about half an hour ; from 7000 to 

 20,000 occur in one cell. They are di- 

 stinguished by Braun as macrogonidia, from 

 other gonidia of smaller size and longer 

 shape, which he calls microgonidia, furnished 

 with four long cilia and a red parietal spot. 

 These have a different history. From 30,000 

 to 100,000 appear in the parent-cell, their 

 development presenting the same character 

 up to the time when the motion begins. 

 Then, the microgonidia, unlike the net-form- 

 ing macrogonidia, leave their parietal posi- 

 tions with a whirling motion, and move 

 through the entire cavity of the parent-cell, 

 until at length the membrane of the latter 

 bulges out in one or more places and bursts, 

 and the microgonidia leave the cavity in a 

 swarm. According to Cohn, they are at first 

 enclosed in a thin mucilaginous pellicle pro- 

 truded before them [like the swarming spores 

 of PEDIASTRUM]. However, they escape, 

 become free in the water, and swim about for 

 a long time. At length they come to rest, 

 sink to the bottom, and remain there heaped 

 in green masses, like cells of PROTOCOCCUS, 

 for a long period. Their farther history is 

 unknown. 



The rapidity of the growth of the Hydro- 

 dictyon-net by the above process is wonder- 

 ful ; the component cells of the net increase, 

 under favourable circumstances, to 600 times 

 their original length in a few weeks. In cul- 

 tivated specimens, the whole history, from 

 the origin of a net to the production of a 

 new one, passes over in three or four weeks. 

 The original size of the cells is about 1-2500", 

 in the fully developed condition they are 

 about 1 to 4'' long. No development of 

 spores or resting-sporeshasyetbeen observed. 



BIBL. Vaucher, Conserves, p. 82. pi. 9; 

 Areschoug, Linncea, xvi. p. 127. pi. 5 (1842); 

 Hassall, Brit. Freshw. Alg. p. 225. pi. 58 ; 

 Al. Braun, Verjungung, fyc. (Rejuvenescence, 

 Sfc.}, Ray Soc. Vol. 1853, passim; Cohn, 

 Nova Acta, xxiii. 207. pi. 19. 



HYDROGASTRUM, Desv. = BOTRY- 



DIUM. 



HYDROMORINA, Ehr. A family of 

 Infusoria. 



The two genera of which it consists, Poly- 

 toma and Spondylomorum, appear to be 



Monads (species of Algae) undergoing divi- 

 sion. See these genera. 



BIBL. Ehrenberg, Ber. d. Berl. Akad. 1848. 



HYDROPHILUS, Geoff. See HYDROUS. 



HYDROPHORA, Tode. A genus of 

 Mucorini (Physomycetous Fungi). Mildews 

 growing on the dung of animals, distin- 

 guished by the indurated persistent peridiole 

 and the conglobated spores. Two species 

 are described as British. 



H. stercorea, Tode. Fleecy ; filaments 

 simple, very long, fugacious, white ; peridioles 

 spherical, yellow, subsequently black. Com- 

 mon on dung after much rain. 



2. H. murina, Fr. Filaments scattered, 

 short, simple, persistent, white ; peridioles 

 yellow, subsequently opake. On rats' dung. 

 (Mucor fulvus, Sowerby, pi. 400. fig. 4.) 



BIBL. Berkeley, Hook. Brit. FL ii. pt. 2. 

 331 ; Fries, Syst. My col. iii. p. 314, Summa 

 Veget. p. 87. 



HYDROUS, Linn. A genus of Coleop- 

 terous Insects, of the family Hydrophilidse. 



H. piceus is one of the largest aquatic 

 British beetles. We have selected the head 

 to illustrate the structure and arrangement 

 of the trophi, &c. in the Coleoptera (see 

 INSECTS). The perfect insect is about H" 

 in length. The full-grown larva is about 3" 

 long ; it has no lateral branchiae, but two 

 filiform branchial appendages at the end of 

 the body. 



BIBL. Westwood, Introduction, fyc. ; Du- 

 meril, Consid. gen. s. I. Insectes; Stephens, 

 British Beetles. 



HYDRURUS, Ag. A genus of Confer- 

 void Algae which we have placed for con- 

 venience among the PALMELLACE^E, but 

 it seems to form a link between these and 

 the ULVACE^E. The frond consists of a 

 branched, feathery, very gelatinous expan- 

 sion, the branchlets set with minute pro- 

 cesses or ramelli (PI. 3. fig. 8 a) ; in the ge- 

 latinous substance are imbedded minute cells 

 with homogeneous green contents, most 

 closely set in the ramelli, more scattered in 

 the older part of the frond (PI. 3. fig. 8 b). 

 H. Ducluzelii, Ag., grows to a length of from 

 1 to 6'', and from 2 to 4'" in diameter, 

 attached to stones in mountain brooks and 

 rivers ; the recent frond is of brownish-olive 

 in mass, green when dried. When fresh it 

 has a very offensive smell. Reproduction not 

 described. 



BIBL. Harvey, Brit. Alg. (1 ed.) p. 180 j 

 Hassall, Br. Freshw. Alg. p. 302. pi. 77. 

 fig. 3 ; Kiitzing, Tab. Phycol. pi. 34. fig. iii. 



HYMENIUM. The term applied to the 



