266 Proceedings of the Royal Irish Academy. 



(e) Edradion. — That the polyps are capable of great retraction is a fact 

 which is of the utmost importance in specific determination. The manner 

 in which this is accomplishetl is very simple. The tentacles are first 

 infolded, the eight lobes of ihe verruca close over them, and then the whole 

 is withdrawn into the coenenchyma. The longitudinal section (fig. 53) of 

 Scirpearia jlagcilum shows the attachment of the strong retractor muscles 

 which accomplish this; and fig. 10 of Juncdla juiuxa shows the polyps 

 completely embedded in the coenenchyma. 



It would be difficult to imagine that such extreme differences as that 

 given in figs. 49 and 51 of S.flftgtUum could occur in one species, were it 

 not for the fact that as great differences actually occur in one indi\idual 

 colony, e.g. figs. 77, 78,79, and 80 of Scirpearia fiircfita. 



This has been discussed in detail under the different species, so that it is 

 neoesBary here to refer only to the actual existence of such a phenomenon. 



(d) Dittributum. — The distribution of the polyps has been used as a basis 

 for generic diagnosis, so that it is essential to study this character in detail 

 and see to what extent the various distinctions can be said to obtain. Wright 

 and Studer (L), p. Ixv, in defining their new genus Scirpearella, make the 

 following statement : — " The polyps are arranged in rows or spirals, retractile 

 with more or less prominent verrucae," thus separating it from Scirpearia, 

 which they describe as ha\-ing " the polyps seated in two longitudinal rows 

 on each side of the stem." 



Hickson, in discussing these, says : — " The genera Scirpearia and Scir- 

 pearella, however, appear to me to be still good genera. The arrangement of 

 prominent verrucae in tiro rows in the former genus and in a spiral manner 

 in the latter, combined with other characters, renders them relatively easy of 

 identification." 



Let us for the present disregard the question of spicules, and consider the 

 group as a whole with regard to this character. 



It must be borne in mind that since the publication of the work of these 

 authors, the species JunctUa trilineata Thomson and Henderson has been 

 esublished ; and the present memoir contains another new species, namely, 

 Scirpmria qnndrxHn/ata. The result of this is that unless the distinction 

 drawn between these two genera is modified, these two species would 

 necessitate the establishing of two new genera to include them. 



The first problem before us then is :— What is the factor underlying the 

 distrOnUion of the terrucae I The answer to this quesrion — namely, the number 

 of main longitudinal canals — has already been discussed. 



The only exception to this rule is what may be termed " the low verruca- 



