106 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 . . 
areas are the mouths of members of the colony. 3B, Open end of same. abundant in the Medi- 
q Sell ley of Botryllus, showing circlets of individuals. », Two ci torranean and else- 
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 (Pyrvosoma 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. 
