Key to the Indian Species of Plumatella.
| I. | Ectocyst more or lessstiff, capable of transverse wrinkling only near the tips of thezoœcia, never contractile or greatly swollen; zoœciarounded[[BF]] at the tip when the polypide isretracted. Free statoblasts elongate; the free portion of theirswim-ring distinctly narrower at the sides than at the ends. | |
| A. | Ectocyst by no meansrigid, of a uniform pale colour; zoœcia never emarginate orfurrowed, straight, curved or sinuous, elongate, cylindrical | fruticosa, p. [217]. |
| B. | Ectocyst rigid;zoœcia (or at any rate some of the zoœcia) emarginate andfurrowed. | |
| b. Ectocyst darkly pigmented overthe greater part of each zoœcium, white at the tip; branching ofthe zoarium practically dichotomous, profuse, as a rule both horizontaland vertical; zoœcia straight or slightly curved orsinuous | emarginata, p. [220]. | |
| b'. Ectocyst colourless andhyaline; branching of the zoarium sparse, lateral, irregular,horizontal; zoœcia nearly straight, strongly emarginate andfurrowed | javanica, p. [221]. | |
| b''. The majority of thezoœcia distinctly L-shaped, one limb being as a ruleadherent; ectocyst never densely pigmented. | ||
| β. Zoœciacylindrical, their furrowed keel never prominent | diffusa, p. [223]. | |
| β'. Zoœcia (or atany rate some of the zoœcia) constricted or tapering at the base,their emargination and furrow conspicuous | allmani, p. [224]. | |
| II. | Ectocyst stiff;zoœcia truncated when the polypide is retracted. Surface ofzoœcia minutely roughened, distinctly annulate on the distalpart | tanganyikæ, p. [225]. |
| III. | Ectocyst swollen andcontractile, capable of transverse wrinkling all over the zoœcium;zoœcia never emarginate | punctata,p. [227]. |
There has always been much difficulty in separating the species of Plumatella, and even now there is no general consensus of opinion as to the number that should be recognized. The difficulty, however, is much reduced if the following precautions are observed:—
(1) If the zoarium appears to be tangled, if the branches intertwine or overlap, or if the zoœcia are closely pressed together, the whole mass should be carefully dissected out. This is necessary not only because zoaria belonging to different species are sometimes found entangled together but also because it is often difficult to recognize the characteristic method of branching and shape of the zoœcia unless it is done.
(2) As large a part as possible of each zoarium should be examined, preferably with a binocular microscope, and allowance should be made for irregularities and abnormalities of all kinds. What must be observed is the rule rather than the exceptions.
(3) When the statoblasts are being examined, care must be taken that they lie flat and that their surface is parallel to that of the nose-piece of the microscope. If they are viewed obliquely it is impossible to see their true outlines and proportions.
(4) In order to see the relative proportions of the capsule and the swim-ring it is necessary that the statoblast should be rendered transparent. This is often difficult owing to the presence of air in the air-cells, but strong nitric acid applied judiciously will render it possible (p. [240]).
In supervising the preparation of the plates that illustrate this genus I have impressed upon the artist the importance of representing what he saw rather than what he thought he ought to see, and the figures are very close copies of actual specimens. I have deliberately chosen for representation specimens of Plumatella preserved by the simple methods which are often the only ones that it is possible for a traveller to adopt, for the great majority of naturalists will probably have no opportunity of examining living specimens or specimens preserved by special methods, and the main object, I take it, of this series is to enable naturalists first to distinguish the species described and then to learn something of their habitat and habits.
Geographical Distribution.—Of the seven species included in this key five have been found in Europe (namely P. fruticosa, P. emarginata, P. diffusa, P. allmani, and P. punctata), while of these five all but P. allmani are known to occur in N. America also. P. javanica is apparently peculiar to the Oriental Region, while P. tanganyikæ has only been taken in Central Africa and in the Bombay Presidency.
Types.—Very few of the type-specimens of the older species of Plumatella are in existence. Allman's are neither in Edinburgh nor in London, and Mr. E. Leonard Gill, who has been kind enough to go through the Hancock Collection at Newcastle-on-Tyne, tells me that he cannot trace Hancock's. Those of the forms described by Kraepelin are in Hamburg and that of P. tanganyikæ in the British Museum, and there are schizotypes or paratypes of this species and of P. javanica in Calcutta. The types of Leidy's species were at one time in the collection of the Philadelphia Academy of Science.