CHAPTER XVII 



THE MINOR COELOMATE PHYLA 



PHYLUM POLYZOAi 



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 re- 

 semble hydroid polyps — in their general shape, size and circle of 

 tentacles. Closer inspection shows that they are triploblastic animals. 

 In the majority of the Ectoprocta each individual consists of two 

 distinct parts, the zooecium or body wall and the polypide^ consisting 

 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. 



The form chosen for illustration, Plumatella (Fig. 416), belongs 

 to an order (Phylactolaemata) of the Ectoprocta in which the lopho- 

 phore is not a simple circle, as is the case in the other order, the 

 Gymnolaemata, but is horse-shoe-shaped. A small ridge, the epistomey 

 overhangs the mouth in this group but not in the Gymnolaemata. 

 The mouth opens into the oesophagus which passes into a capacious 

 stomach with a caecum which is attached by a strand of mesoderm, 

 the 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 trans- 

 ported through the whole of the alimentary canal by cilia. 



The body cavity (also in all Ectoprocta) is a true coelom containing 

 a colourless fluid, and the cells which line it give rise to the germ 

 cells. Polyzoa are hermaphrodite; the testes are formed on the 

 funiculus and the ovary on the body wall. When the germ cells are 

 ripe the so-called ifiter tentacular 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 



^ Often called Bryozoa. 



