186 ZOOLOGY. 



The Polyzoa are divided primarily into the Entoprocta, 

 {Loxosonia and PedicelUna) in which the vent is situated 

 within the circle of tentacles, and the Ectoprocta, in which 

 the vent lies outside of the lophophore — a group comprising 

 all the higher Polyzoa {Gymnolmmata and PliylactolcemMa). 



The development of the Polyzoa is not very complicated. 

 In the marine forms, as studied by Barrois, the germ passes 

 through a morula stage ; after which the cells are arranged 

 into two halves, separated by a crown of cilia ; at this stage 

 it is called a blastula. At the time of birth the ciliated germ 

 is a disk-shaped gastrula, with two opposite faces or ends, 

 separated by the crown, one (aboral) bearing in its centre 

 the mouth-opening. This ciliated free-swimming top-like 

 gastrula stage is called a trochosphere. 



After swimming about as ciliated larvae (trochospheres), 

 the shell or ectocyst develops, and the larva becoming station- 

 ary, the cystid forms, its calcareous shell develops, and finally 

 the polypide is indicated, and the primitive cell is gradually 

 formed. 



As seen in PJialangella Jlabellanf!, the larva, after becoming- 

 fixed to some object, consists of a white pyriform mass, 

 closely enveloped by an ectocyst, with numerous fat globules 

 between the latter and the white mass. The ectocyst swells, 

 into a discoidal sac, with endocyst, ectocyst, and an external 

 zone, while the internal whitish mass transforms into the 

 polypide. The discoidal sac formed by the endocyst consti- 

 tutes simply the basal disk of the primitive coll. The future 

 opening of the cell appears on the upper surface of the cell. 

 The budding out of the secondary cells of the polyzoarium 

 or corm then takes place. It begins by the appearance of a 

 cell placed in front and below the primitive cell, and which 

 borders it on each side ; its secondary cell then divides into 

 two, each of which successively gives origin to three cells, 

 and we thus arrive at an Idmonea stage ; and finally the 

 Phalangella stage is reached, the process being a dichoto- 

 mous mode of budding quite analogous to that which pro- 

 duces the broad, flattened corm of Eschar ina. 



The levelopment of Membranipora pilosa, which is very 

 abundai.t on our shores, growing on sea-weeds, is of singu- 



