206 ROBERT PAYNE BIGELOW ON 



gin, so that there is a complete sheet of endodenn separating the subumbrellar from the 

 exumbrellar mesogloea. 



Musculature. The exumbrella is devoid of muscles, but on the opposite side 

 there is a continuous sheet of muscle fibres, which is spread over the subumbrella, 

 except a narrow zone at its margin, and is continued over the oral arms to their finest 

 ramifications, and also into the subgenital cavities. 



Most of the fibres on the subumbrella do not take an evenly circular course, but are 

 undulating. They form in this way a series of double " arcades " like those found by 

 Haeckel in other species of Cassiopea. There is one of these double arcades for each 

 interrhopalial space. The surface of the mesogloea in this region is grooved. The 

 sheet of muscle fibres lies directly upon it, and is therefore corrugated, the grooves 

 being parallel with the fibres. 



The muscular layer upon the oral arms is smooth, and its fibres take a longitudinal 

 course, extending to the digitella and oral vesicles. 



In the subgenital cavities the arrangement of the muscle fibres could not be made 

 out; but their presence was revealed by the squirming movements of the thin membrane 

 that separates the subgenital cavity and bears the gonads and gastric filaments. 



Structure of the Marginal Sense Organs. Each rhopalium has a pigment spot 

 on the aboral side near the extremity, and each one lies in a deep sensory niche. The 

 dorsal sensory groove, common in the Pelagidae, Aurelia, etc., is entirely lacking; 

 although Keller found in C. polypoides a slightly depressed thickening of the ectoderm 

 that corresponds to it. The sensory niche and rhopalium are, with the exception of the 

 pigment spot, similar in all essential particulars to those found in Pelagia. The 

 rhopalium is the only organ in the sensory niche (Fig. 56). It is a hollow, finger-like 

 projection attached by its base to a low ridge that runs along the roof to the central wall 

 of the niche. This ridge is penetrated longitudinally by the continuation of a radial 

 canal from the stomach, and the lumen of the rhopalium opens into the distal end of this 

 canal. In the distal half of the rhopalium the lumen is nearly obliterated by the increase 

 in thickness of its endodermal lining. Here the endoderm, instead of being a columnar 

 epithelium as elsewhere, is a mass of parenchyma-like cells, each of which contains a 

 large calcareous concretion, a so-called otolith. A thin, supporting membrane separates 

 the endoderm from the ectoderm. At the distal extremity of the rhopalium the ecto- 

 derm is a thin, cuboidal epithelium, while over the rest of the surface it is a thick, sensory 

 epithelium resting on a thick network of fine nerve fibres. This, in turn, rests on the 

 supporting membrane. I have observed no ganglion cells in this layer of nerve fibres, 

 which is continued under the epithelium of the rhopalial ridge to the central wall of the 

 niche, where it becomes imperceptible. There are no thickened bands of these fibres 



