PART I. 

 GKNKRAL MORPHOLOGY. 



In any living- coral, be it a .simple or colonial form, the .soft polyp above can be readily distin- 

 guished from the hard, calcareous skeleton below. The latter is generally cup shaped, and serves 

 as a support and protection to the former. Structural!}' the polyp is very simple, and is either 

 distinct or united with others. While alive it is variously colored, and assumes very different 

 appearances according as it is fully expanded or retracted within its calice. When expanded it 

 presents two distinct regions — a smooth column, generally cylindrical in outline, and terminated 

 distally bj' a more or less flattened oral disk. In the center of the latter is the slit-like mouth, 

 while toward its periphery are one or more cj'cles of simple or knobbed tentacles. Sometimes 

 the polyps, instead of being distinct and independent, retain but partial individuality, and 

 give rise to complicated discal, tentacular, and columnar systems. 



Upon decalcification the nearly colorless basal or aboral region of the polyp becomes 

 exposed. This is generally cylindrical or conical, and A^ery comjjlex in detail, being deeply 

 grooved obliquely or vertically, and otherwise invaginated in correspondence with the skeletal 

 projections; terminally it may be truncated or tapering. 



The interior of the polyp is hollow, but much subdivided by two series of vertical parti- 

 tions, arranged in cycles. The members of one series — the mesenteries — hang from the 

 body wall, their tree edge provided with a filamentous organ, except above, where some unite 

 with the stomoditum depending from the margin of the oral aperture; the other partitions — the 

 septal invaginations — are wedge-shaped inturnings of the basal wall, which are occupied by the 

 skeleton, and are arranged so as to alternate with the mesenteries. Invaginations of the 

 basal wall may also occur centralh', when thej' are usuall\' connected with the septal inturnings. 

 The mesenteries cease before the aboral termination of the poh'p is reached, while the septal 

 invaginations are best developed below, and distally never extend the whole length of the 

 expanded polyp. 



Microscopically the body wall is constituted throughout of three distinct layers, very different 

 in character. The outer comprises various glandular, protective, and sensory elements; the 

 middle is a nearly homogeneous, jell^'-like substance; while the inner is mainly constituted of 

 glandular and muscle cells, and is often loaded with unicellular algee, the so-called zooxanthell^. 



On any colony new polyps, originating either as buds or by division of some other polyp, 

 are to be found in various stages of growth. Within the mesenterial mesogloea of the mature 

 poljqis maj' occur groups of sexual cells, and within the polypal cavity may be free larvae 

 undergoing the early phases of development. Such are the broad features chai-acteristic of 

 Madreporarian poh^ps, and these will now be described in greater detail. 



COLUMN WALL. 



As comparatively few coral polyps have been described from their appearance in the fully 

 expanded condition, the descriptive term cohnnn has been but little emploj-ed in ^Madreporarian 

 literature, though of universal recognition in woi'ks on the Actinia? for the corresponding region. 

 When coral polyps are fully expanded their coluninai- character is usually very obvious, but in 



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