STYLASTERIDAE 



is reduced on the hind surface of the colony; what may be the cause of this our imperfect knowledge 

 of the biological conditions of the Stylasteridac does not enable us to explain. 



If we grind down the branches to about the median longitudinal plane we are able to see the 

 pores in the whole of their length (PL III, figs. 19 and 20) and it becomes 

 apparent that these have a typical bend in their course, curving inwards 

 towards the longitudinal axis of the branch and then downwards towards 

 its base. The pores especially the gasteropores are of varying length; 

 the terminal pores are, as a rule, quite short and much shorter than those 

 we find further in on the branches. This indicates a terminal and centrifugal 

 growth in the branch and gives us the key to the understanding of the bent 

 course of the pores (Text-fig. A). The pores are first formed at or near the 

 tip of the branch and approximately in its longitudinal direction ; during the 

 growth of the branch the pore is gradually moved down on its cylindrical 



part. The plane of the opening of the pore lies at each place approximately Text-fig. A. Diagram showing 



the arched development of the 

 at right angles to its longitudinal axis and the latter will therefore become pores ; n piwbothras symnutri- 



curved during the growth of the branch , as will be seen from the ac- "" durin g the 8 rowth of tte 



colony, 

 companying diagram. 



A few of the gasteropores open into large, irregular, longitudinal canals, which are formed 

 secondarily in the central part of the branch ; often however their base is formed of one or two tabulae. 

 In their original form the gasteropores are cylindrical with a slightly expanded basal part in which 

 the zooid is attached; but during growth the gasteropores often assume a more irregular appearance. 

 The dactylopores also are of varying depth; they are cylindrical, more or less curved and have a 

 slightly constricted opening region; the pore aperture itself, as mentioned, is situated on the top of a 

 slightly prominent, almost wart-shaped protuberance. The calcareous substance of the colony is 

 penetrated longitudinally and transversely by fine canals in which the stolons lie. No regular arrange- 

 ment could be noticed in these fine canals, neither in Pliobothrus symmetricus nor in our other North 

 Atlantic Stylasteridae. 



The connection of the pores and canal system in Pliobothrus symmetricus is of special interest, 

 when we compare with the genus Steganopora established by Hick son and England (1905, p. 26). 

 The open communication, which is not so very seldom observed between the pores and an irregular 

 central canal system in Pliobothrus, is said by Hickson and England to be a constant and always 

 occurring character in Steganopora and here also results in an open and direct communication 

 between the gasteropores and the dactylopores. Such a communication also occurs irregularly in Plio- 

 bothrus symmetricus and we cannot consider this as a fundamental difference between this species and 

 Steganopora spinosa, as maintained by Hickson and England. In reality there is here only a gradual 

 difference, which by no means entitles us to make a generic distinction and the last-mentioned species 

 must therefore be referred to the genus Pliobothrus. 



Moseley (1881) has shown, that the structure of the zooids in Pliobothrus symmetricus is very 

 characteristic. The gasterozooid (Text-fig. B) has a fairly broad basal part, from the circumference of 



