HABITS, ETC., OF OEREBRATULUS LACTEUS. 161 



until its transformation into the adult form. During this 

 time its food supply is varied. It consists chiefly of very 

 small particles of the refuse material always found in sea 

 water; in no instance were living organisms found in the 

 digestive organs. The cilia on the inner surface of the ento- 

 derm keep up a constant current inward through the single 

 opening at the centre of the oral surface^ and another inter- 

 mittent current outward. The former runs along the anterior 

 side of the invagination and carries with it the food particles. 



In the stomach the current becomes more or less rotary, 

 carrying the food round rapidly until all that is digestible 

 has been absorbed. The refuse material is caught from time 

 to time in the intermittent current and carried outward along 

 the posterior side of the invagination, and discharged by the 

 same opening at which it entered. This single opening, the 

 blastopore, thus functions as both mouth and anus. Often 

 the refuse is thrown to quite a distance by a vigorous flap 

 of the lappets^ the larva at the same time being driven 

 forward with a jerk. After the primitive intestine becomes 

 differentiated into stomach and oesophagus the opening 

 between the two is strongly constricted by a sphincter muscle, 

 which operates exactly like the pyloric sphincter in our own 

 bodies. 



Morphology of the Pilidium. — As the larva elongates 

 in passing from radial to bilateral symmetry two enlarge- 

 ments appear, one on either side of the blastopore, growing 

 downward from the border of the oral surface. These form 

 ear-like lappets, diverging slightly from each other, and the 

 larva has now assumed the helmet form of a typical pilidium. 

 This species is slightly elongated antero-posteriorly, and the 

 lappets are quite thick (figs. 9^ 10). 



The whole pilidium is built up of the three primitive 

 tissues nearly unmodified. The ectoderm forms the skin, the 

 entoderm the intestine walls, while the mesoderm fills the 

 space between. The gradual development and transforma- 

 tion of these three tissues takes place very slowly, and affords 

 a good opportunity for studying the tissues themselves and 



VOL. 43j PART 1. — NEW SERIES. L 



