igoy.] Records of the Iiidniii Mitscuni. 283 



The bilateral symmetry thus established is further emphasized 

 by the presence of what appears to be a strand of muscle-fibres 

 which occupies the central axis of each half. These fibrous 

 strands are clearly seen in both longitudinal and transverse sec- 

 tions. They commence below, in the very base of the hydranth, 

 which is largely made up of them, and pass upwards through the 

 spherical cells of the endoderm nearly to its apex. Throughout 

 their length they distribute fibres among the peripheral endoderm 

 cells in the manner shown in figs. 3 and 4. Under a y 2-inch 

 objective longitudinal sections of these structures show a fibrillated 

 appearance which is well defined, and the individual fibres can be 

 seen distributed between the spherical endoderm cells. These 

 fibrils do not show nuclei. There can be little doubt that they 

 have a muscular function. 



The simple arrangement shown in the transverse section (fig. 

 i) in which the hydranth is divided into two separate halves, 

 each of which contains a muscle-strand, was found in the prox- 

 imal part of every individual examined in section, but in their 

 distal parts the number of these " mesenteries " is increased. 

 Some sections show three, others four or more. Figure 2 is of a 

 somewhat oblique section showing four such mesenteries. At the 

 base of the hydranth the specialised endoderm ceases abruptly in 

 a sharply-defined line, which can be readily seen in optical section 

 (pi. xvi, fig. 4). The central cavity is continued below into a tube 

 of small dimensions composed of somewhat delicate cells. This 

 tube, which usuall)^ has the form of a dice-box, its calibre being 

 smallest in the middle of its length (fig. 4), opens into a long 

 straight tube with thick walls composed of regular columnar cells. 

 The other end of this straight tube opens into one of the irregular 

 endodermal spaces of the coenosarc. 



The Basal Plate or Coenosarc — • 



The basal plate is so closely attached to the skin of the fish 

 that on removing a portion of it an outer layer of the fish's skin 

 is often detached with it. In structure the plate is not the same 

 throughout its whole extent. As a whole it is very like the 

 attachment plate or coenosarc of Hydraciinea (4), but without 

 the strong chitinous element so characteristic of that genus. 

 Throughout most of its extent it is composed of two layers of 

 ectoderm widely separated by irregular tubules and spaces with 

 endodermal walls which communicate with one another freely and 

 form a complex labyrinthine structure. The outer layer of ecto- 

 derm does not everywhere pass over this endodermal labyrinth 

 in a smooth and unbroken fashion, but dips down between the 

 layers of endoderm in places, and occasionally the cuticle is carried 

 along with the ectoderm into the same situation. Although most 

 of the ccenosarc has this complex structure, parts of it show the 

 more primitive type consisting of an open meshwo k of irregular 

 trabeculse, each of which is a tube composed of two layers — 

 ectoderm and endoderm with an external cuticle. 



