REPORT ON THE PENNATULIDA. 127 



the canals themselves being lined by a single layer of epithelial cells. 

 In the septum dividing the two lateral canals from one anotlier is 

 contained, as will be described more fully below, the calcareous axial 

 rod or stem (Fig. 3, c.) 



The function of the whole canal system of Pennatulida is a matter 

 of much uncertainty. The meshes of the spongy connective-tissue 

 communicate freely with the cavities of both polypes and zooids, and 

 also, according to Kolliker, with the main canal system of the rachis 

 and stalk. The fluid in this system is probably a nutritive one, mixed, 

 however, very largely with sea-water ; and the well developed 

 muscular system may be supposed to have for its main function the 

 maintaining, by compression of portions of the spongy connective- 

 tissue meshwork, of currents from one part of the Pen to another, 

 and in part to effect the slight movements of the leaves described by 

 many writers, notably by Dalyell, who says that " the animal has 

 also much control over the dimensions, reciprocal position, and 

 direction of the lobes,'"* i.e., leaves. 

 •2.— The Stem- 

 As in Funiculi iia, the stalk and rachis are traversed by a central 

 firmly-calcitied stem (P^ig. 8), situated, as shown in Fig. Sc, in the 

 middle of the septum dividing the two main lateral canals from one 

 another. We have investigated the structure and anatomical relations 

 of the stem in two of the specimens of Pcnnatula from Naples referred 

 to above. 



The first of these specimens has a total length of 4Jin., whereof the 

 stalk forms the lower Igiu., and the rachis the remainder. The rachis 

 bears twenty-seven leaves on each side, each of the larger ones being 

 composed of eleven polypes. The stem (Fig. 8 c) is thickest at the 

 point of junction of stalk and rachis, at which place it has a diameter 

 of 0"044in. A point of considerable interest is that at this spot the 

 stem is very distiiactly quadrangular in transverse section, the sides 

 being even slightly concave, as in Fuuiculinn. This quadrangular 

 shape of the stem of Peuuatula appears to have been hitherto very 

 generally overlooked. From the point mentioned the stem e.xtends 

 down to the bottom of the stalk, preserving its quadrangular character 

 until very close to the bottom where it becomes cylindrical. Its 

 diameter diminishes at first very gradually, but in the bottom half- 

 inch very rapidly. On reaching the bottom of the stalk it is bent 

 back on itself, so as to form a hook, the loop of the hook being in 

 contact with the bottom of the stalk, and the upturned limb of the 

 hook, which is ^in long, being extremely slender and only very 

 imperfectly calcified. The extreme tip is bent back a second time 

 towards the lower end of the stalk for a length of aliout iin. 



In the rachis the stem loses its quadrangular character almost 

 immediately, becoming cylindrical ; its transverse section being 

 circular or somewhat oval (Fig. 3 c). It tapers gradually in passing 



* Dalyell, op. cit., p. 19-2, 



