394 
Laomedea, Thujaria Thuja. When growing in Sertularia, the filaments to a great extent 
fuse together into the membranes first described by Macnus. In the tubes of 
-Tubularia they may for long stretches remain separate, 
consisting of long cylindrical cells and giving rise to 
numerous, often opposite, divaricate branches (fig. 331). 
Specimens growing in the test of Laomedea never fus- 
ed into membranes; the filaments growing in the soft 
substance between the outer and the inner membrane 
of the wall were curled and consisting of long narrow 
cells, while the filaments situated near the surface 
were usually composed of broader and shorter cells 
(fig. 332). In these specimens foldings of the membrane 
were not observed probably owing to the soft consistence 
of the medium, while they are frequently met with in 
the filaments growing in the firmer tubes of Sertularia 
Fig. 331. and Tubularia. MAGNus (l. c. p. 67) explains these fold- 
OOH OT 2 RAD CEE ings as caused by the surrounding cells of the same 
Growing in the tube-wall of Tubu- 5 ag N s x 
Jaria, 310.:1. species hindering the extension of the membranes in the 
places where the filaments are densely crowded. The 
foldings are, however, also produced when the filaments are growing separately, as 
f. inst. in the tubes of Tubularia 
(fig. 331), in which case the fold- 
ings must be caused by the re- 
sistance of the chitinous membrane 
in which they grow. As empha- 
sised by Kuckuck (1. c. p. 23), the 
penetration of the filaments into the 
membrane of the Hydroid must 
take place by means of an en- 
zyme dissolving the chitine, but 
this enzyme is probably only se- 
creted by the young cells, and the 
older cells are therefore only able 
to realise their growth by folding 
their walls when imbedded in 
chitine on all sides. As shown by 
Kuckuck (1897) the cells contain 
several ribbon-shaped chromato- 
phores (comp. fig. 333). 
The free fertile filaments are 
always short in the specimens 
Fig. 332. 
\ = 2 å 5 = Rhodochorton membranaceum, growing in the test of Laomedea at 
growing in Sertularia pumila, 7-84 Hirsholm. On the right a tuft of sporangiferous filaments, 310 :1. 
