BY W. A. HASWELL, M.A., D.SC. 1033 



the mouth, wliile in the neighbourhood of the posterior end of the 

 slit the anus is formed at a somewhat later stage.* 



When the process of invagination commences the larva is 

 covered uniformly with cilia; soon, however, the anterior or 

 cephalic end of the embryo loses its cilia, but becomes surrounded 

 just in front of the mouth by a strong prae-oral ciliated band. 

 The epiblast of the cephalic end becomes thinner than the rest 

 except in the centre, where a thicker group of cells remains, destined 

 to give rise to the cerebral ganglion. 



The embryo becomes more elongated, assuming the shape of a 

 pear, the broad end being the head and the narrow end the tail. 

 From the centre of the former there grow out in most instances 

 one, or sometimes two, long and slender motionless flagella ; bvit 

 these are frequently absent. The alimentary canal, though still 

 simple in form, has now become densely ciliated internally, and 

 undergoes frequent strong contractions. In sections made through 

 an embryo at this stage, towards the end of the second day, a few, 

 apparently irregularly placed, cells are to be found between the 

 epiblast and the hypoblast; these probably form the foundations 

 of the middle layer, 



In the course of the third day the alimentary canal becomes 

 differentiated into distinct esophageal, gastric, and intestinal 

 regions. The prse-oral circlet of cilia becomes elevated on a dis- 

 tinct slightly oblique ridge, and a reniform eyespot becomes 

 developed at a little distance from the ganglion, and connected 

 with the latter by a fibi'ous strand. A thin- walled vesicle makes 

 its appearance on the third day at the posterior exti-emity of the 

 body, and soon attains a considerable size ; it is apparently formed 

 by involution of the epiblast, and remains connected with the 

 exterior by a pore at the side of the anus. 



*Conn (Zool. Anzeiger, VII.) describes the blastopore in Serpula as 

 becoming elongated and closed, the mouth and anus arising at its two ends. 

 Salensky (I.e.) describes the mouth in Psygviobratichus as formed indepen- 

 dently of the blastopore after the closure of the latter. In Spirorbis Gotte 

 ^"Zur Entwick. der Wurmer" Zool. Anz. 1881, p. 189) states that the 

 blastopore becomes converted into the mouth. 



