DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
783 
(fig. 173, st.) which forms the first rudiment of the oesophagus, and is therefore a 
stomodseum. The lamella (si.) is pushed inwards for some distance so as to form a kind 
of pouch, filled with a solid mass of ectoderm. The lamella at the bottom of the 
stomodseum then becomes perforated, so that the ectoderm becomes continuous with 
the entoderm, though there is still no cavity in the oesophagus. The process is funda¬ 
mentally like the formation of the stomodseum in the axial polyp, but the mouth¬ 
opening is differently formed. The cavity of the oesophagus appears as a funnel- 
shaped depression at the inner end of the stomodseum opening within into the gastric 
cavity of the axial polyp and terminating outwardly in the solid plug of ectodermic 
tissue which forms the stomodseum. The cells of this plug are small and rounded, 
without definite arrangement. Farther inwards the ectoderm cells assume an 
irregularly columnar arrangement on either side of the cavity, and at the inner end of 
the stomodseum become definitely columnar and graduate insensibly into the entoderm 
cells around the lips of the oesophagus. 
The cavity of the oesophagus soon breaks through to the exterior, forming a small 
oval opening, the mouth, which gradually becomes elongated in the dorso-ventral 
plane until it has the form of a long cleft. The cells of the stomodseum become at 
the same time of a high columnar form, and cilia make their appearance at the inner 
ends of those on the ventral side. By the action of these cilia strong currents are 
drawn into the colony through the mouths of the buds which are held widely open. 
These currents may be readily shown by adding finely-powdered carmine to the 
water, when the particles may be seen to be sucked with force into the mouths of 
the buds. In this manner large quantities of water are sucked into the cavity of the 
axial polyp, whose body may thus become greatly distended. When a sufficient 
amount of water has been taken in, the mouths of the buds are tightly closed, and 
the water is thus retained. The water thus taken in is kept in active circulation 
by means of the cilia which cover the entoderm. The currents, which are rendered 
plainly visible by the particles suspended in the fluid, follow a definite course. In the 
upper chamber of the peduncle the current sets always backwards, and the fluid flows 
thence into the lower chamber through the openings along the sides, a.nd at the 
posterior end of the peduncular septum. In the lower chamber the current flows 
forwards into the anterior part of the gastric cavity. 
It is by means of the fluid contained in the gastric cavity that the young polyp is 
enabled to effect the active creeping movements which it now performs. The modus 
operandi is as follows. The anterior part of the body being well distended, an active 
peristaltic contraction of the circular muscles takes place and the fluid is thus forced 
backwards into the posterior region (which may now be termed the peduncle). The 
latter consequently becomes much elongated, somewhat as the ambulacral “ foot ” of 
an Echinoderm is protruded, and the body is pushed forwards a short distance. The 
circular muscles then relax and the longitudinal ones contract in such a manner as to 
pull the posterior region forwards towards the anterior part which adheres to the 
5 h 2 
