CCELENTERATES, SPONGES, AND PROTOZOA 



i6i 



interesting as exhibiting division of physiological labour between 

 the members of the colony, a very common phenomenon in the 

 animal kingdom. As regards nutrition we can distinguish be- 

 tween individuals which 

 serve as stomachs, and 

 slender mouthless indivi- 

 duals richly provided with 

 nettling-organs and con- 

 stantly fishing for prey. 

 Each of the larger feed- 

 ing individuals is sur- 

 rounded by a circle of 

 the fishers, which provide 

 it with food. As all the 

 members of the colony 

 are connected together 

 by the common flesh, 

 anything which is caught 

 and digested benefits all 

 the neighbouring polyps 

 more or less. 



Compound Jelly Fish. — Division of labour, however, is 

 carried to a far greater extent in another group, that of the 

 Compound Jelly-Fish {^Siphonophora), including a large number 

 of translucent free-swimming forms such as Diphyes, Physophora 

 (fig. 416), and Velella. 



Each of these is in effect a floating colony produced probably 

 by the budding of an originally single jelly-fish, the shape of 

 the colony being determined by the method of budding, whether 

 from the under side of the original umbrella or from the surface 

 of the much-elongated handle, which is converted in many cases 

 into a long trailing stalk. The members of the colony, or it 

 may even be the organs of certain members, are specialized in 

 all sorts of ways to perform different functions. One may be a 

 gas-containing Jloat, others may be swimming -bells, and so on, 

 but those which are of special interest in the present connection 

 are trumpet -like feeding-individuals which receive and digest 

 food for the common good of the colony, and slender often 

 branching fishing-lines which trail in the water and capture prey. 

 A good example is the Portuguese Man-of-war (Physalia), which 



Fig. 415.— Group of individuals from a Millepore Coral [Millepord] 

 (enlarged). A feeding individual surrounded by five fishers 



VOL. II. 



43 



