378 POLYZOA [CH. 



The body-cavity is regarded as truly coelomic. It contains a 

 fluid in which cells float ; it is traversed by the funicle and often 

 also by numerous strands of mesodermic tissue. The funicle may 

 be the remains of a median mesentery which once separated the 

 coelomic sacs of the two sides. Some of the cells of its walls give 

 rise to reproductive cells, and the body-cavity opens to the exterior 

 in certain individuals which possess an ovary by a short tubular 

 duct, the so-called inter-tentacular organ. This functions as 

 an oviduct and has been regarded by some authors as a modified 

 excretory organ. A portion of the body-cavity is separated from 

 the rest by a horizontal septum, and forms a space at the base of the 

 tentacles into which the cavity of each tentacle opens. This may 

 open into the other part of the coelom or may be completely shut off 

 from it. 



No heart or blood-vessels are present. It is possible that some 

 of the nitrogenous waste matter may be got rid of by means of the 

 inter-tentacular organ, but it has also been suggested that these 

 waste products are stored up in certain cells on the funicle. From 

 time to time the tentacles, alimentary canal and nervous system 

 of an individual undergo degeneration and form a brown mass, 

 called the brown-body, which forms a conspicuous feature in the 

 colony. After a time the body-wall, which has not disintegrated, 

 forms a new set of digestive organs and the brown-body may come 

 to lie in the stomach of the reconstituted individual. Thence it 

 passes to the exterior through the anus. It is thought that much 

 of the waste matter which has accumulated in the body of the 

 animal is, in this way, eliminated. In certain Phylactolaemata 

 there is a pair of small coelomiducts which probably serve as 

 excretory organs opening between the mouth and the anus. ;:J 



A nerve-ganglion lies between the mouth and anus, situated in that 

 part of the body-cavity which runs round the base of the lophophore. 



Polyzoan colonies are usually hermaphrodite. The testes are 

 as a rule formed by the multiplication of the coelomic cells which 

 form part of the body-wall, while the ovary originates from the 

 funicle or from the body-wall. They may be found in the same 

 individual or in different individuals of the same colony. As a 

 rule the eggs develop within some part of the parent colony, but 

 they may be laid, escaping from the body-cavity through the inter- 

 tentacular canal, and then they pass at once into the sea-water. More 

 usually the early stages of development are passed through in the 

 tentacle-sheath or in a special pouch called an ooecium (Fig. 177), 



