POLYZOA. 



551 



body-cavities of the zooecia are in wide communication, a rudimentary 

 polypide bud is formed near the ovum. This passes round the egg 

 and invests it, but remains rudimentary. In the Ctenostomata the 

 eggs develop in the tentacular sheath or in the sea. ^ 



The body -wall consists of cuticle, ectoderm, and a delicate 

 layer of cells which line the body- cavity. The peritoneal cells 

 are ciliated in the Phylactolaemata, in which group there is also 

 a thin muscular layer in the body-wall. 



The mouth is placed at the anterior end of the body in the 

 midst of the circlet of ciliated tentacles, and the disc bearing it and 

 the tentacles is called the lophophore (Fig. 440). The lophophore 

 is either circular (Gymnolaemata), or is drawn out into two lobes 

 (horseshoe-shaped lophophore, Fig. 441, Phylactolaemata), and the 

 tentacles are set along its 

 edge. The latter are simply 

 hollow processes of the body- 

 wall; they are provided with 

 longitudinal muscles, and 

 their cavity communicates 

 with a circular canal which 

 surrounds the body at their 

 base. This canal communi- 

 cates with the body-cavity 

 in the Phylactolaemata, but 

 is separate from it in the 

 Gymnolaemata. The tenta- 

 cles serve both for procuring 

 food (setting up by means 

 of their cilia whirlpools in 

 the water) and for respira- 

 tion. 



The digestive organs lie 

 freely in the body-cavity, 

 and are attached to the body- wall by a mesodermal reticulum which 

 traverses the body-cavity. The term funiculus is applied to one 

 or two special strands (Fig. 440, F) of this tissue. The mouth is 

 placed in the centre of the circular or horseshoe-shaped lophophore, 

 and a movable epiglottis-like process, called the epistome, is in the 

 fresh-water forms placed on the dorsal side of it, and projects 

 over it. 



The alimentary canal is bent on itself, and consists of (1) an 



FIG. 441. Anterior part of the body of Lophopus 

 (from Lang, after Allman) from the right side. 

 t tentacles cut off near the base ; o mouth ; ep 

 epistome ; si pharynx ; ga ganglion ; an anus ; 

 pr rectum. 



