286 



PHYLUM ANNELIDA 



by an internal skeleton. The nervous system lies in the ectoderm — a 

 very primitive character, and consists of a ring round the mouth, and 

 of a cord down the left side of the body. An interesting point is the 

 presence of a closed vascular system with nucleated red cells. The 

 body cavity is well developed, and is divided into chambers. The 

 sexes are united ; and the larva, known as Actinotrocha, undergoes a 

 remarkable metamorphosis in the course of its conversion into the 

 adult. 



Class PoLYZOA or Bryozoa 



As usually defined, the class includes two sub-classes, the Ectoprocta 

 and the Entoprocta, but it seems almost certain that these are distinct 

 classes. 



The Ectoprocta include fresh-water and marine forms, in which the 



Fig. 155. — Diagram of an Ectoproctous Polyzoon 



{PUimatella). 



L., Lophophore ; PH., pharynx ; A., anus ; S., stomach ; 

 M., retractor muscle ; F., funiculus, a cord of mesodermic 

 tissue ; O., cells that form " statoblast " buds ; B., an 

 ordinary bud ; £., epistome over the mouth ; T., ten- 

 tacles ; S/f., outer wall of zooecium. 



anus is outside the bases of the tentacles. The nervous system is repre- 

 sented by a ganglion placed between the mouth and anus. There is a 

 body cavity. There is no vascular system. Nephridia are absent. All 

 are colonial and bud very freely ; the marine forms show considerable 

 division of labour among the members of the colony. 



