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PERCY SLADEN TRUST EXPEDITION 



large number of specimens they do not occur at all and the presence of centric or excentric 

 canals is certainly not an essential feature of the structure of the branches of the genus 

 Polytrema or of its species P. miniaceum. The statement that the branches of Polytrema 

 consist of 3 — 4 joints (gliederartigen Abschnitten) made by Merkel and confirmed by a 

 good figure, is not of general application. I have examined a very large number of 

 specimens from the Mediterranean Sea and from other localities and I have not yet been 

 fortunate enough to discover a single one that corresponds with this description. The 

 chambers of the arm of a Polytrema are very variable in shape and size and they 

 communicate freely with then neighbours of the same row or stratum. Their outlines 

 are indicated only by the pillars (Text-fig. B P). At the free end of a branch there may 

 be seen a variable number of openings. In the diagrammatic text-illustration B, I have 

 shown four such openings. Of these three may be regarded as homologous with the 



A. 



B. 



Fig. 1. Diagrams to illustrate the structure of the growing points of A. Sporadotrema, B. Polytrema, 

 C. Homotrema, c incomplete chambers, c completed chambers, i interlocular spaces, P pillar pores of 

 Polytrema. 



openings of incompleted chambers and the fourth (i) as the opening of an interlocular canal 

 or space. The incompleted chambers have walls perforated by foramina on the sides that 

 are free and external as in Sporadotrema, but unlike Sporadotrema the walls separating 

 one chamber from another that lies internal to it are also perforated by foramina. 



The wall of a chamber that separates it from an interlocular space is imperforate. 



At the growing point of a Sporadotrema (text-illustration A) there is, as a rule, only 

 one circle of chambers enclosing an irregular interlocular space. In Polytrema on the 

 other hand there may be a cluster of chambers, two, three or more deep on one side, and 

 one or two deep on the other, enclosing an excentric or possibly in some cases a centric 

 interlocular space. The pillars P represent the distal and proximal sides of chambers of a 

 row and their cavities are really interlocular spaces which may be continuous with the walls 

 of the deep seated interlocular canals. 



The characteristic feature of the branch of a Polytrema is that the chambers appear 

 to be very irregular in outline and this is due to fusion of neighbouring chambers which 



