PEARL PRODUCTION. 19 



(p. 79, 1904). Although most of those which we have examined are not surrounded 

 by any pearl, there can be no reasonable doubt that these are the parasites that form 

 the nuclei of the orient pearls. When we compare SHIPLEY and HORNELL'S figures 



Fig. 3. Young larval Cestode (Tetrarhynchus, sp.) encysted in connective tissue of pearl oyster. 



(Part II., Parasites, Plate I., figs. 4 to 6 and 13) with SEURAT'S figures (GiARD, loc. 

 cit., figs. 1, 2 and 3) and our present figures (Plate II., figs. 17 to 22 and Plate III., 

 figs. 1 to 8) there can be little or no doubt that these all represent similar stages 

 in the same kind of organism. We do not mean that our larvse necessarily belong to 

 the same species as SEURAT'S. In fact, differences in size and details of structure 

 convince us that they are not identical, but the resemblance is sufficiently close to 

 indicate that they all belong to allied organisms. 



Moreover, it is clear that these are all larval Cestodes in the blastocyst condition 

 containing young scolices. It was the possession of calcareous corpuscles noticed in 

 the fresh condition in 1902 in the Gulf of Manaar that caused us first to identify 

 these larvse as Cestodes. We now enumerate as Cestode characters : 



1. The invagination to form the head of the adult worm ; 



2. The hooks upon portions of the invaginated surface ; 



3. The calcareous corpuscles in the walls of the vesicle ; 



4. The division of the (? muscular) tissue on the floor of the invagination into 



several masses (probably four, as either two or three can usually be seen in 

 different views). 



The invagination (Plate II., figs. 17, 20) agrees very closely with the " figures 

 ideales " of early stages of the genus Tetrarhynchus given by P. J. VAN BENEDEN 

 (" Vers Cestoides," pi. xxiii.), and with the sections of the larvae of Rhyncho- 

 bothrius adenoplusius. PINT., from Lophius, showing receptaculum and developing 

 scolex, published in the third part of his ' Studien tiber Tetrarhynchen ' (Taf. ii., 



D 2 



