XCVl INTRODUCTION. 



this stage (according to Joliet) it is fertilized by passing 

 spermatozoa^ and immediately enters on its further deve- 

 lopment. At this time its behaviour is in many respects 

 that of a polypide. Thanks to the muscles which it has 

 borrowed^ it now rises to the entrance of the cell and now 

 retreats to its recesses, until^ having assumed its perfect 

 larval form, it passes through the sheath into the water. 

 The " auxiliary polypide " has discharged the functions 

 of an ooecium, and has both supplied the ovum with a 

 brood-chamber and a way of escape. In its leading par- 

 ticulars, this marvellous history rests on the authority of 

 three able observers. 



The true marsupium seems to be confined to the Chei- 

 lostomata, and even amongst them is by no means uni- 

 versal. In other sections of the class ova are frequently 

 developed in special receptacles, and not in the zooecia : 

 to these I have given the names yonoecium and gonocyst^. 

 Of the former we have a good example in the genus 

 Crisia f ; it also occurs amongst the Cheilostomata, and 

 in one family at least [Alcyouicliidce) of the Ctenostomata. 

 The gonocyst has only been noticed in certain Cyclosto- 

 matous genera; it is probably only a modification of 

 the gonoecium. Of the history of these structures, 

 however, we know little ; they remain to stimulate and 

 reward further research. 



I do not propose to follow the ovum through the stages 

 of its development ; nor shall I enter minutely into the 

 numerous modifications of the larval form. To do so 



* See Terminology, page iii. 



t Sraitt has investigated the gonoecium of Crisia, and considers the pro- 

 diiction of ova in it to be asexual. ' Oiu Hafsbryozoernas Utvockliug,' &c. 

 (186.")), p. 19. For a critical notice of Suiitl's observations, see Barroi.s, 

 ' Embryologie ' (1877), p. 59. 



