1O2 



ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



fact which has had much to do with the course of evolution in 

 certain species. Reference should be made to vol. i, pp. 474-481, 

 where some of the Colonial Zoophytes are figured and described, 

 and also to vol. iii, pp. 327-328, for a brief account of the life- 

 histories of such colonies. We are here only concerned with the 

 characteristic features of colonial life, and it will be convenient to 

 consider separately the two sub-groups of Sea- Flowers (Anthozod) 

 and Hydroids (Hydrozoa). 



Colonial Sea- Flowers (Anthozoa or Actinozoa). To students 

 of the British fauna the most familiar Sea - Flowers are the 

 solitary forms known as Sea -Anemones, which abound on our 



shores. But in war- 

 mer parts of the globe 

 Corals are equally 

 abundant, and these 

 may be either solitary 

 or colonial. The 

 former, or cup-corals, 

 may be compared to 

 anemones, but the 

 lower part of the 

 body is supported by 

 a limy skeleton, while 

 the latter may be re- 

 garded as colonies of 

 cup-corals, and present wide variations in shape, according to 

 the mode of growth. In the majority of cases the members 

 of the colony are all alike (fig. 1090), but this is not invariably 

 the case. For in some of the Eight-rayed Sea-Flowers (Octac- 

 tinid), e.g. the Sea- Pen (Pennatula), some of them are devoid of 

 tentacles, and participate neither in active feeding nor in the 

 production of egg-cells. Their special duty appears to be that 

 of promoting breathing by setting up currents of sea-water which 

 circulate through the fleshy substance of the colony. Ciliary 

 action is the agency employed. 



Hydroids (Hydrozoa). The branching or encrusting colonies 

 known as Hydroid Zoophytes exemplify division of labour more 

 or less. As we have elsewhere seen, some of the individuals 

 are specially concerned with egg-propagation, and these may be 

 liberated as little free-swimming jelly-fish or medusae (see vol. iii, 



Fig. 1090. Small Colony of a Coral (Astroides calycularis) 



