tENNATULA PHOSPHOREA. 33 



II well-developed layer of lon^jitudinal muscles, having the same 

 arraugemeut as in the stalk ; and underneath this a connective-tissue 

 layer which differs considerably from that of the stalk, for instead of 

 forming a dense compact layer it has the character of a loose spongy 

 meshwork, traversed by large irregular canals and passages freely 

 opening into one another and into the canal system between the folds 

 of the longitudinal muscles. 



At the sides the structure of the wall between the several pohpe- 

 leaves is much the same as that just described on the dorsal and 

 ventral surfaces, with the exception that the longitudinal muscle layer 

 is absent, and the spongy connective-tissue layer consequently thicker ; 

 but opposite the bases of attachment of the leaves it is very different. 

 As sliown on the left-hand side of Fig. S, the wall is here reduced to 

 a thin connective-tissue membrane, separating the bottoms of the 

 polype cavities from the main dorsal and lateral canals. 



The partitions separating the main canals from one another are, 

 as in the stalk, formed by prolongations of the connective-tissue layer ; 

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

 In the septum dividing the two lateral canals from one another 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 KoUiker, 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 FunicuUna, the stalk and rachis are traversed by a central 

 firmly-calcilied stem (Fig. 8), situated, as shown in Fig. 3 c, 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 Fcnnatula from Naples referred 

 to above. 



The lirst of these specimens has a total length of 4iiu., whereof the 

 stalk forms the lower Igin., 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 



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



