36 



ESSENTIALS OF BIOLOGY 



of animals, Hydrozoa, etc., is that of a simple sac attached to 

 some fixed object. The sac has one opening, or mouth, and this 

 is fringed by tentacles. The sac, or coelenteron is the nutritive 

 cavity and food materials are taken into it by the tentacles, while 

 indigestible debris is expelled via the same orifice. 



I id. The Racemose-Hydra Type. In a vast number of 

 kinds of animals, (very many of the Coelenterata) the individual 

 organism consists of a great number of segmental zooids, each of 

 which has the essential structure indicated above. 



Zooids—^ 



Coerwsourc 



Atvlls- 

 IntestVne 



l/T-^ 



P ,.''Jfi^drothjeccie 



^\ ~ 2 



Fig. 6. 



I, A Hydrozoon ; 2, the same, only two Zooids being shown ; 3, the radial symmetry of 

 the Zooid, being the sac with its mouth-orifice and radial tentacles ; 4, a Zooid belonging to 

 the Polyzoa : here the symmetry is bilateral (there being mouth and anus) and radial (because 

 of the tentacles). Racemose symmetry of the colony is shown in i. 



\ie. The Polyzoan Type. This is represented in No. 4 of 

 Fig. 6. The zooid in the Hydra type is essentially a sac built 

 up of two main layers — the outer ectoderm, which is protective 

 and aggressive (having stinging cells) and the inner endoderm 

 which is nutritive in function. The symmetry of the zooid is 

 radial. But in the animals called Polyzoa the middle layer of 

 the body wall — the mesoderm — is much more highly difi"erentiated 

 than in the Hydra type, and the symmetry of the zooid is now 

 bilateral, or right-left in respect of some of the organs. 



11/. The Colonial Types. Changing our point of view, 

 we now look upon many of the multicellular animals as being 

 compounded of units, or segments, arranged in the racemose form 

 indicated above, as well as in other forms. Thus the Hydrozoa, 

 or Zoophytes are made up of zooids enclosed in little cups, or 

 thecae (No. 2 of Fig. 6) and these thecas are arranged in branching 

 forms, one of which is indicated in No. i of Fig. 6. Corals are 

 examples of colonies of zooids of the Hydra type. Polyzoa are 



