DR. E. B. WILSON ON THE DEVELOPMENT OF RENILLA. 
725 
surface but lying nearly horizontally at the margin, where new polyps continually 
make their appearance. Each polyp may be retracted into a “ cell ” which is morpho¬ 
logically the basal part of the polyp and forms a part of the disc. 
The polyp has eight septa or mesenteries, eight pinnate tentacles, and eight mesen¬ 
terial or gastric filaments, of which the dorsal pair are more slender and of different 
structure from the others. The two lateral pairs of septa bear the reproductive 
organs, male or female, as the case may be, the sexes being separate. The septa are 
provided with delicate longitudinal muscles by which the retraction of the polyp into 
its cell is effected. These muscles are always placed on the ventral sides of the septa, 
so that the dorsal gastric chamber contains no muscles, while the ventral chamber 
contains them on both sides. Thus we observe a marked bilateral symmetry in the 
arrangement of all the internal organs, which is further emphasized in the dorso- 
ventral elongation of the mouth and oesophagus. This symmetry is expressed also in 
the arrangement of the calyx-teeth, which are conical projections from the walls of the 
gastric chambers at the level of the upper face of the disc. The ventral chamber is 
always destitute of a tooth, the dorsal chamber always bears one, and the lateral teeth 
are symmetrically arranged with respect to the dorso-ventral axis. 
Besides the large sexual polyps there are other forms known as the rudimentary 
individuals, or in Kolliker’s terminology, as the zooids. These are microscopic in size, 
have no tentacles, no mesenterial filaments, no reproductive organs, and commonly 
only two calyx-teeth—those, namely, on the ventro-lateral chambers. The zooids 
possess, in fact, only septa, mouth, and oesophagus, the latter being richly ciliated 
within. Two distinct forms of zooids exist. One of these is represented by a single 
large zooid, placed near the middle of the disc on the dorsal side, and provided with 
the full number of calyx-teeth. It is for the most part through the mouth of this 
zooid that the water is discharged which circulates through the cavities of the colony. 
For this reason I shall call it the exhalent zooid, a name which seems preferable to 
Kolliker’s term “ Haupt zooid.” The other zooids are arranged in groups or clusters 
on the dorsal sides of the polyp-cells in the median line ; there are usually four such 
groups on each cell. It is their function to draw water from the exterior into the 
cavities of the colony, as may be shown by adding finely pulverised carmine to the 
water of the aquarium. Minute but powerful currents may thus be seen setting into 
the open mouths of the zooids. The zooids, like the sexual polyps, exhibit a marked 
bilateral symmetry in the disposition of all their organs ; the mouth and oesophagus 
are elongated in the dorso-ventral plane, the gastric chambers are of different sizes 
and symmetrical arrangement, and the two calyx-teeth occupy corresponding positions 
on the sides of the median plane. 
The colony as a whole is also bilaterally symmetrical to a very striking degree. 
This is more obvious in young specimens, but is always clearly marked even in the 
largest colonies. Each polyp has its counterpart on the opposite side of the colony, 
and the dorso-ventral axes of the two polyps have the same inclination to that of the 
MDCCCLXXXTII. 5 A 
