258 INVERTEBRATE MORPHOLOGY. 



of each other (Pedicellinci), or unite together to form a single 

 tube and open into the vestibule, between the epistonie and 

 the ganglion. 



2. Order Ectoprocta. 







The order Ectoprocta includes the great majority of forms 

 which are referable to the class Polyzoa. They are without 

 exception colonial forms of small size in which the tentacles 

 are arranged either in a circle or in the form of a horseshoe 

 surrounding the mouth, the anus being invariably situated, 

 contrary to the arrangement in the Endoprocta, outside the 

 limits of the lophophore. The tentacles, too, when retracted 

 are not flexed or coiled as in the Endoprocta, but are simply 

 approximated to form a bunch, each tentacle being straight 

 and parallel to its fellows. 



The most characteristic peculiarity of the Ectoprocta, 

 however, is the power which they possess of withdrawing or 

 retracting the anterior portion of the body with its crown of 

 tentacles within the posterior part (Fig. 115). This latter 

 portion is enclosed in the ectocyst (ec) to which the body- wall 

 is closely adherent and which forms a chitinous or, in some 

 cases, more or less calcareous cell, termed a zooecium. At the 

 mouth of the cell the cuticle becomes suddenly exceediugly 

 thin, so that the anterior portion of the body is quite mobile, 

 and by means of special retractor muscles (TO) may be with- 

 drawn within the zooacium. The retraction is a process of in- 

 vagination, similar to what occurs in the withdrawal of the 

 pharynx of the Annelida, the most anterior lophophoric part 

 of the retracted portion not, however, sharing in the invagina- 

 tion : the whole process indeed is similar to what may be 

 obtained when one finger of a glove is caught from within 

 somewhat less than half way from the tip and drawn down 

 towards the palm ; half of the lower portion will thus be in- 

 vagiuated within the other half, while the tip of the finger 

 remains uninvagiuated. 



The portion of the body-wall enclosed by the ectocyst is 

 thin, its longitudinal muscles being for the most part sepa- 

 rated in the form of bands traversing the coslom and function- 



