Zoarium. The zoaria are circular or nearly so except when about to undergo division, in which case they are constricted in the middle. As a rule they measure nearly an inch (2 cm.) in diameter. The polypides have a definite arrangement in each zoarium, being divided into four groups, each of which has a fan-like form. In the first place they are separated into two main divisions in a line running through the centre of the zoarium, and secondly each main division is separated into two subordinate ones in a line running across the other at right angles. The number of zoaria joined together in a single compound colony is very variable; sometimes there are only about half a dozen and sometimes several hundreds. The common investment in living colonies is often as much as two inches thick and has a translucent dark greenish colour due to the presence in it of green cells.
Fig. 47.—Pectinatella burmanica.
A=polypide with the lophophore expanded, × 15; a=œsophagus; b=cardiac limb of stomach; c=stomach; d=rectum; e=anus; f=funiculus. [The muscles are omitted and the external tubercles are only shown on part of the polypide. The specimen is from the Sur Lake, Orissa.] B=statoblast from Ceylon, × 35.
Polypide. The polypide can be extruded for a distance of at least 5 mm. Its whole external surface is covered with minute tubercles. There are about 90 tentacles, which are long and slender, the velum at their base being narrow and almost straight. The stomach is of considerable stoutness.
Statoblast. The statoblasts are of large size, measuring from 1 to 1.75 mm. in diameter. In form they are almost circular, but one side is always slightly flattened. The marginal processes are very short and bear a single pair of hooks at the tip. The capsule is circular and small as compared with the free part of the swim-ring.
Type in the Indian Museum.
P. burmanica is evidently a near relation of P. gelatinosa, Oka, from Japan, differing from that species in the shape of the statoblasts and in having much longer tentacles. The arrangement of the polypides in the zoarium and the general structure of the statoblasts are very similar in the two species.
Geographical Distribution.—P. burmanica was originally described from a swamp at Kawkareik in the Amherst district of Tenasserim but has also been found in the Sur Lake near Puri in Orissa. Dr. A. Willey obtained specimens from a pool by the roadside between Maradankadewela and Galapitagala, at the foot of Ritigala, N. Central Province, Ceylon.
Biology.—The first specimen obtained was a statoblast fixed to a tube of the oligochæte worm Aulophorus tonkinensis taken at Kawkareik in March. At the same time young zoaria, which did not yet possess a common investment, were found on a leaf growing on a twig which drooped into the water. Large compound colonies were taken in Orissa in October. They completely encased the stems of reeds, thus forming hollow cylinders, but slipped from their supports when the reeds were pulled out of the water. In life they resembled gelatinous algæ rather than animals and exhibited a striking similarity to masses of zoaria of Lophopodella carteri surrounded by such algæ. Some of the colonies were evidently dying and contained few polypides in a living condition, but many statoblasts; others were in a flourishing condition and were producing larvæ and statoblasts simultaneously.