416 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



pxert a mutual pressure upon one another. The deeper parts of the layer are characterized by 

 the presence of patches of finely granular pigment matter, arranged closely or somewhat distant 

 from one another. In the areas of greatest concentration the granules extend almost to the 

 periphery of the ectoderm, but they are mainly internal to the nuclear zone. Probably they are 

 to l>e regarded as of the nature of pigment granules, and are to be distinguished from the granules 

 of glandular cells. They are manifestly the chief cause of the general opacity of the body wall 

 in many fissiparous species. (See also, Ixi>]>h>/ll!tt, PI. XVII. fig. 122.) 



MESOGLCEA. 



The mesogloea of coral polyps has generally been described as a perfectly structureless layer, 

 without any of the migrant connective-tissue cells, such as are characteristic of the mesogloea of 

 the greater number of Actinian polyps. The homogeneous condition is found in many of the 

 species here described, especially where the polyps are small, but in others it becomes somewhat 

 more complex. The layer stains feebly, or not at all, and when perfectly homogeneous and 

 transparent may be indistinguishable from the clear field of the microscope. 



In large polyps, such as Tsophyllia dipsacea, and also in Mseandrina, the mesogloea is rather 

 thick, and minute connective-tissue cells occur sparsely throughout. In sections the cells are 

 circular or oval in shape, with a central nucleus, and minute prolongations extend in all direc- 

 tions: many of these reach one or other of the surfaces of the layer, and there come into con- 

 tact with the ectodermal or endodermal cells. In some instances the processes extend right 

 across from one layer to the other, but are mostly disposed in an irregular stellate manner. 

 Their close connection with the ectoderm and endoderm would seem to indicate their origin from 

 line or both layers, except in the mesenterial mesogloea, where obviously they can be derived 

 only from the endoderm. 



The mesogloea is usually of uniform character and consistency throughout any polyp, but a 

 slight difference is revealed in preparations of Tsophyllia dipsacea, which have been stained with 

 borax carmine and methyl blue. The layer is colored a bright blue, lint narrow tube-like por- 

 tions, which scarcely take up any coloring matter, stretch across the layer, or in other sections 

 appear as small, light-colored disks; with hematoxylin it remains unstained, and exhibits no such 

 differentiation. 



The ectodermal and endodermal surfaces of the mesogloea are mostly even, hut in some 

 regions, especially on the face of a mesentery which bears the longitudinal musculature, the 

 surface becomes folded, or may even form complicated branching pla'itings, so as to afford an 

 increased area for the muscular fibrils (PI. XVIII. fig. ISO). The endodermal surface in the 

 uppermost region of the column may also be deeply folded for the same purpose (PI. XVII, 

 fig. 121). In no case, however, has the musculature been found to become actually embedded 

 within the mesogloea of the column, such as occurs among anemones where a strong mesogloea! 

 sphincter is formed (Sagartidse). 



As the mesogloea is practically alike in structure throughout the tissues of any polyp, it will 

 lie unnecessary again to refer to il in detail in describing the individual organs. Along the line 

 of attachment of the mesenteries to the skeletotrophic tissues, and less frequently elsewhere, 

 peculiar mesoglceal processes occur which seem to serve as a. means of attachment of the polypal 

 tissues to the skeleton (PI. XIII. rig. 95). They are fully referred to on page 4-81. 



ENDODERM AND SPHINCTEB MUSCLE. 



Gland cells, both in the clear and granular condition, are the main constituents of the 

 lendoderm. Supporting cells are less numerous than in the ectoderm, while the musculature is 



" In a preliminary note, "On the Anatomy of a supposed New Species of ( 'cenopsammia from Lifu," Mr. Stanley 

 Gardiner proposes the name "skeletogloea" for the structureless lamella or jelly of the Actinozoa, instead of a 

 "makeshift term," such a "mesogloea." The introduction of this new term would undoubtedly lead to great confu- 

 sion if employed in the literature of skeleton-producing polyps, while such lias never been the case with Bourne's 

 term, now universally adopted. "Skeletogloea" would have served aptly for the jelly-like, homogeneous matrix in 

 which the skeleton is laid down (p. 483). In his fuller paper (1900, p. 358 I, i fardiner prefers to use the term "struc- 

 tureless membrane" or "basement membrane." 



