MNNATULA PHOSPHOREA. H5 



The stem consists of a dense fibrous matrix, in wliich the fibres 

 are mainly concentric, but partly radial, impregnated with calcareous 

 salts. Unlike Funicttlinii, the central part of the stem is as firmly 

 calcified as the exterior. 



The (Quadrangular character of tho stem iu the stalk is of interest, 

 as it has hitherto been very generally considered diagnostic of 

 i'ltniculiim, which however, unlike PcnimtuJu, preserves the quad- 

 rangular form in the rachis as well. 



Concerning the proportions of tho stem at different parts of its 

 length, the remarks that we have already made when considering 

 FunicuUna* apply also to Pcnnatuhi, the proportions being precisely 

 those which, mechanically considered, adapt it most pex'fectly to the 

 erect posture with the stalk planted in the mud of the sea bottom. 

 We shall return to this point subsequently. 



3. — The Poly pea and Zooids. — 



The differences between the two kinds of individual animals, 

 polypes and zooids, composing the colony, are far more marked in 

 Pennatula than in FunicuUna, owing mainly to the fact that instead of 

 both polypes and zooids being inserted separately into the rachis, the 

 polypes are fused together to form the leaves, while the zooids, as in 

 FunicidiiM, are planted independently of one another. 



The structure of one of these leaves is shown in Fig. 3. Each 

 leaf is triangular in shape, having a short base by which it is attached 

 to the side of the rachis, and long dorsal and ventral borders. The 

 leaf consists of a number of polypes placed side by side and fused 

 together along nearly the whole of their length, the oral ends 

 alone being free. It is important to realise this fully, and to 

 avoid the very common error of speaking of the polypes as "borne 

 on or by the leaves ;" the leaves simply consist of the polypes, each 

 one of which is directly attached to the rachis. 



The free oral ends of the polypes are situated along the dorsal 

 border of the leaf : and each polype, as is clearly shown m Fig. 3, 

 extends down to the rachis and is separately inserted into it. The 

 consequence of this is that the several polypes composing a leaf are of 

 very different lengths, the ones whose mouths are ueai-est the median 

 plane of the whole Pennatula being very short, while those whose 

 mouths are at or near the apex of the leaf are of much greater length. 



It will further be seen from Fig. 8 that while the base of the 

 triangular leaf is formed by the lower ends or bases of the several 

 polypes, and the dorsal border by their free oral ends, the ventral 

 border is formed exclusively by the most ventrally situated of the 

 component polypes, which is also the longest of the whole set. The 

 dorsal and ventral borders of the leaf are not quite straight but 

 curved as shown in the figure. 



* Suj)ra, 11. 5. 



