vni ROTIFERA, POLYZOA, AND BRACHIOPODA 181 



2. THE POLYZOA 



The Polyzoa are an extensive class of animals for the most part 

 marine, which, from the general form that they assume, are readily 

 mistaken for hydroid zoophytes (Hydrozoa, p. 91). They occur as 

 fixed colonies, the form of which varies greatly, supported by an exo- 

 skeleton which is sometimes gelatinous, sometimes chitinoid, sometimes 

 calcareous. They either encrust rocks, forming little patches, or they 

 grow up with a plant-like habit. Most usually the colony is a branch- 

 ing, plant-like structure, though it may assume other forms. The 

 whole consists essentially of a number of minute chambers, or zocecia. 

 as they are termed, each formed by the exoskeleton of one of the 

 zooids. Each zooecium (Figs. 101 and 102) has an aperture, sometimes 

 capable of being closed by a lid or operculum, through which the oral 

 extremity of the zooid is capable of being protruded. At this pro- 

 trusible oral end of the zoqid is a circular or horseshoe-shaped ridge or 

 lophophore bearing a number of simple, slender, ciliated tentacles 

 (jtent). In many Polyzoa the colony bears a series of remarkable 

 appendages, the avicularia, of the nature of modified zooids. A typical 

 avicularium (Fig. 101, avic) has very much the appearance of a bird's 

 head supported on a very short stalk, with a movable part representing 

 the lower jaw, which becomes separated from or approximated to the 

 part representing the upper jaw by movements which closely resemble 

 the movements of opening and closing of the bird's mouth. These are 

 probably defensive organs. The mouth (ino) is a large aperture in the 

 middle of the oral extremity within the lophophore : the anus is situated 

 near it, but outside the lophophore. The digestive canal is a U-shaped 

 tube, divided into pharynx (/>), stomach (stom), and intestine (ini), 

 suspended within a wide body-cavity. There is no vascular system, and 

 the central part of the nervous system consists of a single ganglion 

 (Fig. 102, **), placed between mouth and anus. The sexes are 

 united, and there is a free-swimming cilia larva. 



Probably allied to the ordinary Polyzoa thus briefly characterised, 

 and usually assigned to that class, are three genera, Pedicellina (Fig. 

 103), Loxosoma and Urnatella, the first two colonial, the third solitary, 

 which, among other special features, have the anus situated within the 

 circlet of the tentacles. These are known as the Endoprocta, as 



