396 



NA TURE 



[February 23, 1905 



it can be gathered. A case in point is given by Prof. 

 Herdman. Tlie Mutuvaratu Paar, which lies to the 

 south-west of Karativo Island, yielded during 1889, 

 1890, and 1891 some 117,000,000 oysters, which realised 

 very nearly 1,000,000 rupees — the only fishery since 

 1814 that has returned so large a sum. The oysters 

 raised during these three years steadily increased in 

 value, those lifted in 1891— the oldest— being by far 

 the most valuable. " But the record," he remarks, 

 " shows the risk there is in trying for the enhanced 

 value by delaying the fishery once the oysters are over 



5 years of age. In 1S91 this bed must have been 



6 years old, and they are described as rapidly dying 

 off, many being dead and putrid." 



There are prospects of a good fishing for next year 

 and 1906, but the results of 1907 and the succeeding 

 years depend largely, it is pointed out, on extensive 

 measures of transplantation being undertaken without 

 delay. This Mr. Hornell will doubtless accomplish. 



Prof. Herdman 's memoir on the anatomy of the 

 pearl ovster adds much to our knowledge of the sub- 

 ject, and contains some valuable observations on the 

 living animal. As an instance of the latter we may 



Vv. I. — Pearl-hshiiig Fleet at «ork on tt e Cli> 



cite his remarks on the functions of the foot. These, 

 he points out, " are three-fold : the distal ventral sur- 

 face subserves locomotion ; the median and posterior 

 parts effect attachment by means of the byssal fibres ; 

 and lastly, on account of the general mobility of the 

 organ, and probably of its sensory nature, the tip is 

 of great use in clearing the gills and mantle from the 

 intrusive particles that cannot otherwise be got rid of. " 



It is concerning the latter function that we would 

 direct special attention here. In the living animal 

 Prof. Herdman has observed the foot " pushed 

 between the gill-plate, and over the inner surface of 

 the mantle gently stroking the surface and insinuating 

 itself into the crevices, thus freeing the parts from any 

 foreign bodies . . . that might cause inconvenience." 



Mr. Hornell observed one oyster, which had sustained 

 an injury to the mantle, pass the foot-tip gently around 

 the edges of the wound so as to work off the particles 

 of dirt collected there. The tip was even passed 

 through the wound to make the cleansing the more 

 thorough. 



Concerning the byssus, it is interesting to notice 

 that the operation of dredging for oysters for trans- 

 plantation in no wise injures the animal when 



NO. 1843, VOL. 71] 



anchored by the threads of which this is composed. 

 Under a great strain these break, and are renewed 

 again within an hour or so, the root of the old byssus 

 being sloughed off. 



Some interesting points concerning gill structure are 

 given, especially with regard to the passage from inter- 

 filamentar junctions by ciliated discs to junctions by 

 organic union. 



With regard to sense organs, the pearl oyster is 

 not very well provided. But a distinct response is 

 shown to the stimulus of light and shadow — " a sensi- 

 bility which may be termed dermatoptic," and appears 

 to be located in the edges of the mantle and the sur- 

 face of the foot, where patches of more or less deeply 

 pigmented epithelial cells are met with. 



All kinds of creatures seem to find the pearl oyster 

 a particularly " toothsome " morsel, man alone ex- 

 cepted, who prefers to make manure of their bodies 

 for the sake of possible pearls contained therein. 



No less than seven different kinds of parasitic worms 

 are now known from the pearl oyster, six of which are 

 new species described in this volume. Of these, only 

 one, a cestode larva (Tetrarhynchtis unionij actor), 

 appears to be concerned in 

 the formation of cyst 

 pearls. This fact is inter- 

 esting, inasmuch as the 

 formation of similar pearls 

 in European mussels is 

 due to the cercaria of 

 trematodes. 



.\s 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 file- 

 fishes and elasmobranchs 

 were the intermediate ver- 

 tebrate 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 

 ■'I I'-'-ir- appear that it enters its 



first host — the pearl oyster 

 — as a free-swimming planaria-like larva, inasmuch 

 as certain larvae of this type, but containing calcareous 

 corpuscles recalling those of cestodes, were taken in 

 plankton, and these bear, in many features, a close 

 resemblance to the earliest encysted larvse found in the 

 pearl oyster. 



It is assumed that these free-swimming forms are 

 tetrarhynchids, though hitherto it has been believed 

 that tetrarhynchid larvae make their way into their 

 first hosts while still encased within the egg-shell. The 

 bothriocephalids have free-swimming larvae, but these 

 are ciliated. That the larvse in question must be 

 tetrarhynchids seems certain, since older larvse, show- 

 ing several stages of development, belonged un- 

 questionably to the genus Tetrarhynchus. 



It was believed that these larvse were next ingested 

 bv file-fishes (Balistes), but it now appears that the 

 tetrarhynchid larvae of Balistes, of which three species 

 are described in this report, are quite distinct forms, 

 distinguished by the presence of a vesicle, which is 

 wanting in the pearl oyster larvae. Further, the more 

 advanced larvae of the pearl oyster have arrived at a 

 later stage of development than the larva found in 

 Balistes. 



