788 
DR. E, B. WILSON ON THE DEVELOPMENT OF REN1LLA. 
and its free portion is what is usually termed the polyp. The latter may be entirely 
invaginated into the former by the action of the longitudinal muscles of the septa. 
The calyx-teeth, which have meanwhile increased greatly in length, are situated just 
at the upper surface of the disc, and when the polyp is retracted they radiate from 
the opening of the cell (see fig. 189). 
The foregoing account of the enclosure of the axial polyp will apply equally well 
to the secondary polyps. On account of the continual appearance of young buds in 
the angles between older ones, each of the latter is bordered by younger polyps on 
each side. The latter gradually extend downwards, and finally meet behind the older 
polyp which at the same time bends upwards, and thus becomes enclosed within the 
disc. This is shown in fig. 188, where p 7 . and p 10 . are already extending behind p\ 
As mentioned at p. 789, the ventral compartment of the polyp never has a calyx- 
tooth, a fact which is rendered conspicuous when the polyps are retracted (see 
fig. 189). When the polyp turns upwards and is enclosed in the disc, the ventral 
chamber necessarily comes to be situated on the outer side of the polyp or away from 
the centre of the group. 
By the union of the lateral lines of buds the outline of the disc is completed at 
front, and the marginal buds now form an unbroken series from one side to the other 
behind the axial polyp. At the posterior part of the disc, however, the outline is 
never completed and a permanent sinus remains in which the peduncle is attached. 
This is due to the cessation of the formation of lateral buds in the posterior angle 
after three or four buds have been formed (see fig. 189). 
§ 16. Formation of organs in the secondary polyps. 
The early development of the bud, including the formation of the oesophagus and 
septa, has already been described; but we have still to consider the development of 
organs in later stages. The bud agrees in the main with the primary polyp, but 
there are certain important differences in the sequence of development of certain 
organs. 
a. Development of the calyx-teeth. 
As the bud grows outwards the outer ends of the chambers grow out into obtusely 
conical projections, which ultimately form the calyx-teeth, though they are at first 
closely similar to tentacles. As observed by Kolliker and Eisen, they are formed in 
definite sequence, and I can in the main confirm the accounts of these authors. This 
sequence cannot be observed in the appearance of the calyx-teeth of the primary 
polyp, and is only obscurely shown in the buds which are first formed. As the 
colony increases in size, however, the sequence becomes very marked, especially m 
the posterior parts of the disc where the calyx-teeth are usually longer than 
elsewhere. 
