io6 



ASSOCIATION OF ORGANISMS THE WEB OF LIFE 



presents at regular intervals a number of flower-like markings, 

 each of which is made up of a circlet of individuals, with their 

 mouths near the tips of the " petals". A small hole in the centre 

 of the flower leads out of a cavity (" common cloaca") into which 

 are discharged the waste products of the members of the group. 

 The surface population of the sea is also partly made up of 

 colonial Tunicates. The Salps, for instance, present two stages 

 in their life-history, one of which propagates by budding, the other 

 by eggs (see vol. iii, p. 422). A large number of the latter stage 



are connected to- 

 gether when young 

 into "chains", which 

 may be regarded as 

 temporary colonies. 

 These ultimately 

 break up into their 

 constituent members. 

 A notable example 

 of a permanent free- 

 swimming Tunicate 

 colony is afforded by 

 the Fire - Cylinder 

 (Pyrosoma, fig. 1095), 



Fig. 1095. A, Fire-Cylinder (Pyrosoma) in side view, the small rounded * _j - - _i 



areas are the mouths of members of the colony. B, Open end of same. abundant in tllC IVleCll- 

 c, Small colony of Botrylliis, showing circlets of individuals. D, Two cir- i i 



ciets, enlarged. terranean ana 



where, and giving off 



a bright phosphorescent light, possibly as a means of protection. 

 The colony is shaped like a hollow cylinder which, in one large 

 species (Pyrosoma gigantea), may be as much as 5 feet long, and 

 possesses a contracted aperture at one end, the other being closed. 

 The external surface is covered with pointed projections of the 

 firm test. The small but very numerous individuals are imbedded 

 transversely in the wall of the cylinder, their mouths being ex- 

 ternal. The large central cavity receives all the products of 

 waste, and is comparable to the common cloaca of a Botryllus 

 circlet. The size of the colony is augmented by budding, and 

 eggs are also produced, which develop into minute colonies that 

 are liberated into the surrounding water, there to grow to their 

 full size. 



