204 ROBERT PAYNE BIGELOW ON 



from the centre of the oral disc to the tip of the arm about equals three quarters of the 

 diameter of the umbrella. But the arms are very contractile, and may be shortened to 

 half this length. The arms are slender and graceful in shape, the mesogloea tapering 

 very gradually to the tips of the finest branches. The branches are arranged alternately. 

 The largest one, which is the first formed, is at a point about two thirds the length of 

 the arm from its base. From this point the branches decrease in size gradually toward 

 the base of the arm, and more rapidly toward the apex. The general outline of the 

 arm, therefore, including its branches, is roughly spatulate. 



The Oral FunneU and Brachial Appendages. — Just below the surface of the 

 oral side of each arm there is a longitudinal tube, the brachial canal, that ramifies to 

 each branch, and finally opens to the exterior by funnel-shaped oscula {os., Fig. 3-f ) at 

 the tips of the numerous ultimate branches, and at many places along tlie course of the 

 tube. The margins of these oscula, or oral funnels, are provided with short tentacle-like 

 projections, the digitella. These are covered by an epithelium containing nettle cells, 

 and each has a gelatinous axis in which there are transverse plates of greater density 

 than the rest of the jelly, and these give the structure the cellular appearance first 

 described by Hamann ('8l) . The epithelium lining the tubes and funnels is 

 ciliated. 



There open also into the brachial canals the lumina of the oral vesicles {v., Fig. 34) . 

 These structures, as already stated in the diagnosis, have their points of attachment in 

 the axils of the branches. All except the smallest are flattened laterally. The smaller 

 ones are oval in outline, the larger ones linear. At one side near the apex there is a 

 cluster of short processes that Hamann has homologized with digitella. 



The Oral Disc. — Although the eight oral arms seem to be placed at equal 

 distances and to be alike, they are morphologically in pairs, each pair being homologous 

 to one of the four lips of a semostomous medusa, — an aurelia, for example. The line 

 that separates two members of a pair is therefore, according to Haeckel's nomenclature 

 a perradius. The brachial canals from each pair of arms, on entering the oral disc, 

 converge and unite into a single radial tube that is continued to the centre of the disc, 

 where it unites with the other three. In this way the course of the tubes on the oral 

 disc forms a pattern that resembles a Maltese cross. The larger central vesicle is 

 attached at the centre of the cross. In a living specimen 11 cm. in diameter this 

 measured 3 cm. in length. There are four other vesicles that most nearly approach the 

 central one in size, and these arise from the radial canals near the junction of the 

 brachial canals, and I have called them, therefore, the radial vesicles. In full-grown 

 individuals there are eight more vesicles upon the oral disc, a little smaller than the last, 

 one on each brachial canal distal to the junction. It is only near the periphery of the 



