160 INVERTEBRATE ZOOLOGY 



complete alimentary tract terminating in an anus near the ante- 

 rior extremity of the body. The position of the anal opening 

 with reference to the circle of tentacles serves as a basis for the 

 discrimination between the two orders (Fig. 80), which show 

 extreme differences in internal structure. In the order Ecto- 

 procta, the anus occurs outside the tentacular ring, while in the 

 Endoprocta it is within the circle formed by the tentacles. The 

 lack of a coelom in the Endoprocta renders their relationship to 

 the other Polyzoa open to question. Some investigators have 

 considered that the Endoprocta possibly bear relationship to 

 the Rotifera. The majority of the Polyzoa are colonial. The 

 individuals of a colony are bound together by an organic con- 

 necting material. The individuals are in many instances cov- 

 ered with a gelatinous, horny, or calcified layer forming an 

 exoskeleton. Polyzoa are most abundant in salt water, though a 

 number of genera appear in fresh water. 



I. Order Ectoprocta 



The Ectoprocta are colonial forms which frequently attain 

 considerable size. Individuals of a colony are variously arranged 

 in branching pattern (Bugula, Plumatella), in fiat mats (Crista- 

 tella), as encrusting layers (Microporella, Cribrillina), or as 

 soUd gelatinous masses (Pectinatella), which may attain a size 

 of a foot or more in diameter. The individual zooids uniting 

 to form these different types of colony assume a number of dis- 

 tinctly different shapes. The lophophore is capable of retraction 

 within the anterior part of the zooid through the agency of special 

 retractor muscles. In addition to the action of this introvert, 

 the entire zooid is frequently able to withdraw into the interior 

 of the colony. In some instances, either an operculum or a series 

 of lobes is drawn into the aperture to close it when the zooid is 

 retracted. 



Modified individuals occur in many Ectoprocta. Avicularia 

 are modified zooids shaped much like a bird's head the beak of 

 which is capable of grasping objects and holding them until 

 they disintegrate. The food fragments are then secured by the 

 tentacles. Vibracularia, another type of modified individual, are 

 long whiplike structures which, in their development, seem to be 

 modified avicularia. 



The funiculus is a double strand of tissues which passes from 

 the bend in the alimentary canal through the coelom to the 



