38 A NEW METHOD FOR GROWING HYDROIDS IN SMALL AQUARIA 



length and its branches 72 mm. The total amount of new growth 

 in the part of the colony under observation amounted to 773 mm. 

 (34 inches), and ninety-nine hydranths in thirteen days. 



The rapid growth of this colony was due to a splendid food supply. 

 The hydranths like large copepods, especially the oily Calanus, and 

 there were no difficulties in obtaining a large supply almost every 

 day. To give the hydranths every chance of catching the copepods, 

 the side of the bell-jar to which the colony was fixed was placed 

 facing a south window, but screened from direct sunlight, and only 

 those copepods which are attracted by a strong light were put into 

 the bell-jar. Consequently the copepods spent tlie day knocking their 

 heads against the glass in the neighbourhood of the hydranths, and 

 many got too close and were captured. During daylight the copepods 

 usually kept near the surface and followed the course of the sun, but 

 at night they could be brought amongst the hydranths by turning a 

 strong beam of gaslight on to the colony. 



I have frequently tried to grow Bougainvillia and some other 

 Hydroids on the system described for Syncoryne, but have never met 

 with a real success. A short stolon would run along the glass and a 

 few hydranths would appear, but they remained in a diminutive con- 

 dition. The new growth lacked vigour, which was evidently due to 

 the want of food. Although there were plenty of copepods in the 

 bell-jar the hydranths rarely caught them. 



The tendency of the copepods to congregate within a small area 

 was not always favourable to the colony, as the zone of the copepods 

 did not always correspond with that of the colony. The introduction 

 of the " plunger" system into bell-jars (described in this journal by the 

 author in 1898) also proved useful for the growing of Hydroids. The 

 currents which the plunger created helped to distribute the copepods 

 more evenly in the water, and other species of copepods which are 

 not attracted by light could be utilized for a food supply. The 

 plunger in its journeys up and down a bell-jar does not set up a 

 current in one direction, but in different directions, so that the 

 copepods are carried hither and thither. It was the quick-changing 

 direction of the current that frequently prevented the hydranths 

 from holding their prey. One current carried a copepod upon the 

 tentacles of a hydranth, and, before the tentacle responded to the 

 touch, another current coming from a different direction would sweep 

 the copepod away. 



It must be borne in mind that the number of copepods or the 

 quantity of plankton which can safely be placed into a bell-jar is 

 strictly limited. Overcrowding soon leads to a heavy death rate, and 

 ultimately to the fouling of the water. If copepods are being used 



