CHAPTER XVII 



THE MINOR COELOMATE PHYLA 



PHYLUM POLYZOA 



Coelomate unsegmented animals, always sedentary and nearly always 

 colonial ; with a circumoral ring (lophophore) of ciliated tentacles , and 

 a U-shaped alimentary canal; usually with a ciliated free-swimming 

 larva; asexual reproduction by budding. 



The ordinary individuals in a colony of polyzoa^at first sight 

 resemble hydroid polyps — in their general shape, size and circle 

 of tentacles. Closer inspection shows that they are triploblastic 

 animals with a perivisceral cavity. Each individual consists of two 

 distinct parts, the zooecium or body wall and the polypide, con- 

 sisting of the alimentary canal, the tentacles and the tentacle sheath 

 (which contains the tentacles when contracted). The polypide can be 

 entirely retracted within the zooecium and, as will be seen below, has 

 a much shorter life than the latter. 



In the form chosen for illustration, Plumatella (Fig. 391), the 

 lophophore is not a simple circle, as is often the case, but is horseshoe- 

 shaped. A small ridge, the epistome, overhangs the mouth in this genus 

 but not in all polyzoa. The mouth opens into the oesophagus which 

 passes into a capacious stomach with a caecum which is attached by 

 a strand of mesoderm, xh^ funiculus , to the body wall. From the upper 

 end of the stomach, the intestine runs to the anus which is situated just 

 outside the lophophore. The food, consisting of small organisms like 

 diatoms and protozoa, is collected by the cilia of the lophophore and 

 transported through the whole of the alimentary canal by cilia. 



The body cavity is a true coelom containing a colourless fluid, and 

 the cells which line it give rise to the germ cells. Polyzoa are herma- 

 phrodite ; the testes are formed on the funiculus and the ovary on the 

 body wall. When the germ cells are ripe the so-called intertentacular 

 organ often appears ; this is a tube which opens within the lophophore 

 and serves for the escape of the genital products. Part of the coelom 

 is shut off from the rest by an incomplete septum, as the ring canal 

 which is prolonged into the tentacles. The intertentacular organ opens 

 internally into this. 



The nervous system is represented by a single ganglion, situated 

 between the mouth and the anus, and many nerves chiefly supplying 

 the tentacles and gut. There are no special sense organs. No trace of 

 a vascular system exists. 



