6 Lightbown, Filaments in the Siphonozooids of Pennatulacea 



filaments would appear to have some connection with the size of the 

 colony, for this species is unusually fleshy. The two single rows 

 of mesozooids may be inadequate to discharge the water from an 

 exceptionally extensive canal system, and therefore the filaments 

 may have been retained to promote efficiency in this respect. 



As Pter. pellucidum is not more fleshy than most species of 

 Pteroeides the same hypothesis cannot be advanced with equal 

 weight. In this species however, according to Kolliker mesenteric 

 filaments are present only on the zooid-plate of the lower surface of 

 the leaves. In the fleshier Pter. Steenstrupii mesenteric filaments 

 are present in the zooids on both sides of the leaf. The two species 

 are similar (if one may judge from Kolliker's figure of Pter. 

 pellucidum (1872, fig. 34, Taf. IV.) in that the siphonozooids of 

 the leaves are not so densely crowded together as in other 

 Pteroeides. If mesenteric filaments were common to all species at 

 some point in the generic phylogeny, those species in which the 

 siphonozooids weje sparsely distributed would possibly tend to 

 keep their filaments functional while mesozooids were evolving ; 

 where large numbers of siphonozooids were present many of these 

 would at once become superfluous and the tendency for the fila- 

 ments to degenerate would be established earlier. This may to some 

 extent explain the presence of filaments in Pt. Steenstrupii, and in 

 some zooids of Pt. pellucidum, the siphonozooids of the latter being 

 more crowded than those of the former, but not so much as in 

 other species. In the absence of any phylogenetic evidence on this 

 matter, any hypothesis can only be put forward very tentatively. 



Throughout the order it may be stated that generally the mesen- 

 teric filaments are present in the more fleshy species and absent in 

 the slender forms. Thus the filaments are absent in Scytalium 

 several species of Virgidaria and Pennatula Murrayi. In Virgu- 

 laria mirabilis where Marshall records the fact that it is the rule 

 to find the top of the colony missing (1882, p. 60) it has been 

 suggested that water is ejected from the open ends of the longi- 

 tudinal canals by the contraction of their walls, these open canals 

 being analogous to exhalent zooids. 



Filaments are present in Funiculina which is a very slender and 

 delicate form, but as there still appears to be a certain amount of 

 doubt whether the undeveloped zooids bearing only dorsal filaments 

 are true siphonozooids perhaps this exception may be left out of 

 consideration. In slender species the colony even when fully dis-* 

 tended can hold but little water, and this could readily be expelled 

 by contraction of the delicate muscles of the rachis, the specialisa- 

 tion of mesenteric filaments being unnecessary. 



The presence of filaments in the comparatively slender form of 

 V. Schultzei mav be accounted for by the consideration that the 



