XI POLYZOA 403 



vessels filled with sea-water, the mouths of which were covered with 

 pieces of linen. The vessels were then placed in tide-pools, and were 

 left there for one or two days. When at the expiration of this time 

 they were examined, numerous Pedicellina larvae were found which 

 had fixed themselves to the Coralline Alga. These were cut out 

 together with small portions of the substratum to which they 

 adhered. The Coralline Alga is softened by decalcifying it with acid 

 alcohol, and then sections can easily be cut through it and the 

 adhering larva. 

 _ The larva fixes itself by the edge of the atrium, which we may term 

 the mantle, the corona being retracted so that its cilia point inwards. 

 The mantle edges grow inwards along the surface of fixation so as to 

 completely close the atrium. The anal cone no longer points down- 

 wards but, as a consequence of the greater growth of the posterior 

 surface of the larva, points obliquely backwards. The atrium 

 becomes divided into a lower portion near the surface of fixation, 

 and an upper portion with which mouth and anus communicate. After 

 the complete separation of these two portions the walls of the lower 

 portion fold inwards, its cavity disappears, and its walls are devoured 

 by wandering cells. The upper section is then divided into a portion 

 near the mouth and a portion near the anus ; these two portions are 

 almost separated by the growth of the epistome and of two lateral 

 folds which grow out from the sides of the atrial cavity. 



As the metamorphosis proceeds the anal division of the vestibule 

 grows rapidly upwards, and at the same time the retracted apical 

 plate and brain (dorsal organ) undergo histolysis. The same fate 

 befalls a good many of the cells lining the larval stomach, they pass 

 into its lumen and are there reduced to structureless globules. The 

 apex of the epistome and the tip of the anal cone likewise undergo 

 histolysis and the cilia borne by the former are lost. When the re- 

 mainder of the vestibule has rotated so far that it is directed upwards 

 the adult ciliated tentacles begin to develop as lateral projections on 

 its sides, and its roof is then broken through by a longitudinal slit ; 

 and so the atrium, now become the adult vestibule, is once more open 

 to the exterior. The mouth appears to become closed by the remnant 

 of the epistome during metamorphosis, but this flap is later perforated. 



With open vestibule, developed tentacles, open mouth and anus, the 

 young Pedicellina has reached the adult stage. Its stalk is formed, 

 as will be seen if this description has been followed, from the lower 

 part of the vestibule which has become solid. The formation of fresh 

 buds takes place from the stalk, and the process is very similar in 

 essentials to what occurs in the formation of buds in the Ectoprocta. 

 A protrusion grows out from the stalk into which mesenchyme cells 

 wander, the whole vestibule and the alimentary canal of the bud 

 arise from an ectodermal invagination, which becomes covered with 

 mesenchyme cells which give rise to muscles and mesodermal 

 organs. 



In endeavouring to give a phylogenetic interpretation of the 



