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it can be gathered. A case in point is given by Prof. anchored by the threads of which this is composed. Herdman. The Mutuvaratu Paar, which lies to the Under a great strain these break, and are renewed south-west of Karativo Island, yielded during 1889, again within an hour or so, the root of the old byssus 1890, and 1891 some 117,000,000 oysters, which realised being sloughed off. very nearly 1,000,000 rupees—the only fishery since Some interesting points concerning gill structure are 1814 that has returned so large a sum. The oysters given, especially with regard to the passage from interraised during these three years steadily increased in filamentar junctions by ciliated discs to junctions by value, those lifted in 1891—the oldest-being by far organic union. the most valuable. “But the record," he remarks, With regard to sense organs, the pearl oyster is “shows the risk there is in trying for the enhanced not very well provided. But a distinct response is value by delaying the fishery once the oysters are over shown to the stimulus of light and shadow—" a sensi5 years of age. In 1891 this bed must have been bility which may be termed dermatoptic," and appears 6 years old, and they are described as rapidly dying to be located in the edges of the mantle and the suroff, many being dead and putrid."
face of the foot, where patches of more or less deeply There are prospects of a good fishing for next year pigmented epithelial cells are met with. and 1906, but the results of 1907 and the succeeding All kinds of creatures seem to find the pearl oyster years depend largely, it is pointed out, on extensive a particularly “toothsome " morsel, man alone exmeasures of transplantation being undertaken without cepted, who prefers to make manure of their bodies delay. This Mr. Hornell will doubtless accomplish. for the sake of possible pearls contained therein.
Prof. Herdman's memoir on the anatomy of the No less than seven different kinds of parasitic worms pearl oyster adds much to our knowledge of the sub- are now known from the pearl oyster, six of which are ject, and contains some valuable observations on the new species described in this volume. Of these, only living animal. As an instance of the latter we may
appears to be concerned in
As to the sequence of hosts called upon to nurse this precious cestode of the pearl oyster to maturity much uncertainty prevails. It was thought that filefishes and elasmobranchs were the intermediate vertebrate hosts, and this will probably prove to be the case.
Certain novel features seem to be foreshadowed in the life-history of this parasite when the chain of evidence is complete.
To begin with, it would Fig. 1.-Pearl-fishing Flee: at work on the Cheval Paar.
appear that it enters its
first host-the pearl oyster cite his remarks on the functions of the foot. These, -as a free-swimming planaria-like larva, inasmuch he points out, “ are three-fold : the distal ventral sur- as certain larvæ of this type, but containing calcareous face subserves locomotion; the median and posterior corpuscles recalling those of cestodes, were taken in parts effect attachment by means of the byssal fibres; plankton, and these bear, in many features, a close and lastly, on account of the general mobility of the resemblance to the earliest encysted larvæ found in the organ, and probably of its sensory nature, the tip is pearl oyster. of great use in clearing the gills and mantle from the It is assumed that these free-swimming forms are intrusive particles that cannot otherwise be got rid of." tetrarhynchids, though hitherto it has been believed
It is concerning the latter function that we would that tetrarhynchid larvæ make their way into their direct special attention here. In the living animal first hosts while still encased within the egg-shell. The Prof. Herdman has observed the foot “pushed bothriocephalids have free-swimming larvæ, but these between the gill-plate, and over the inner surface of are ciliated. That the larvæ in question must be the mantle gently stroking the surface and insinuating tetrarhynchids seems certain, since older larvæ, showitself into the crevices, thus freeing the parts from any ing several stages of development, belonged unforeign bodies . . . that might cause inconvenience. questionably to the genus Tetrarhynchus.
Mr. Hornell observed one oyster, which had sustained It was believed that these larvæ were next ingested an injury to the mantle, pass the foot-tip gently around by file-fishes (Balistes), but it now appears that the the edges of the wound so as to work off the particles tetrarhynchid larvæ of Balistes, of which three species of dirt collected there. The tip was even passed are described in this report, are quite distinct forms. through the wound to make the cleansing the more distinguished by the presence of a vesicle, which is thorough.
wanting in the pearl oyster larvæ. Further, the more Concerning the byssus, it is interesting to notice advanced larvæ of the pearl oyster have arrived at a that the operation of dredging for oysters for trans- later stage of development than the larvæ found in plantation in no wise injures the animal when Balistes.
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