422 MEMOIRS OF THE NATIONAL ACADEMY OF SCIENCES. 



upon living colonics of Siderastrsea the tentacles of the expanded polyps at once close upon them 

 and prevent their escape. 



The detailed arrangement of the tentacles presents many differences in the various species 

 studied. As seen externally, the twelve tentacles of Madrepora and Porites (Pis. I and IV) admit 

 of no proper distinction into an inner and an outer series, though varying somewhat in size, and 

 may therefore be described as acyclic or monocyclic; the apical polyps of the former genus 

 hear only a simple cycle of six equal tentacles. Although forming only one cycle, the twelve 

 tentacles in both genera represent two orders, constituted of six entotentacles and six alternating 

 exotentacles. The tentacles of such genera as Orbicella, Solenastrsea, Ocvlvna, Cladocora, and 

 Astrangia, whose asexual method of reproduction is by gemmation, usually exhibit a regular 

 hexameral multieyclic arrangement, with the formula 0, 6, 12, 2-t, etc. Very often the first and 

 second orders are arranged so as to form only one inner cycle of twelve members, with which 

 the twelve members of the second cycle alternate; the third cycle of twenty-four alternates 

 with both these, and so on, according to the number of cycles developed. Where only twenty- 

 four tentacles are present they usually appear as an inner and an outer cycle. 



Very often the hexameral sequence of the tentacles is not complete, especially in < ladocora 

 and Astrangia. The total number of tentacles in mature polyps of Gladocora arbuscula 

 varies from thirty-two to thirty-six, whereas the complete hexameral plan would require forty- 

 eight as the next number after twenty-four has been reached. In describing below the 

 development of the later tentacles of polyps, it is found that the organs do not arise a complete 

 cycle at a time, but in simple or double pairs on each side of the median axis, and in many 

 species a tentacular cycle once commenced is not always completed before the polyp attains its 

 full size and growth ceases. In such a case it is clear that any intermediate number of tentacles 

 between the commencement of a cycle and its completion may be present. 



Whenever an entoccelic tentacle appears, a corresponding exocoelic member usually arises, 

 either simultaneously or shortly after, so that the number of exotentacles comprising the outer 

 cycle is always equal to the sum of the entotentacles of all the inner cycles. Hence in endeav- 

 oring to establish the cyclic scheme of any hexameral polyp, in which the number of tentacles 

 may be intermediate between twenty-four and forty-eight, or forty-eight and ninety -six. 

 incompletion must be looked for in the two outermost cycles, not in the outermost only. A 

 polyp of C/adoconi with thirty-two tentacles bears the cyclic formula 6, 6, 4, 16; one with thirty- 

 six tentacles the formula 6, 6, 6, 18; with forty tentacle- (>. (>. S. 20, where the numbers 16, 18, 

 and 20 represent the exotentacles. The exocoelic cycle thus increases by the same amount as 

 the outermost entoccelic cycle. The order of appearance of the tentacles demonstrates that it is 

 impossible to establish hexameral completion for all the cycles until the outermost is reached, and 

 then relegate any omissions to this, as is usually attempted. Am* omission due to hexameral 

 incompletion affects both the exocoelic and the last entoccelic cycles. 



The members of any tentacular cycle are nearly always alike in size, but the tentacles 

 usually show a diminution in length in passing from the inner to the outer series, a condition 

 expressed by the term entacmaeous. The organs in Madrepora and Pontes are exceptional in 

 that they vary in size in a very definite manner in the same cycle, while Orbicella radiata offers 

 a marked exception to the entacmaeous order. The polyps of this species do not readily expand 

 their tentacles, and hence are not always favorable for observation. When fully extended the 

 different tentacular cycles are found to be widely apart, and the twelve members comprising the 

 first cycle are much smaller than those of the next, and even less than the tentacles of the 

 outermost cycle. The tentacular plan of a polyp of 0. radiata, with three hexamerous cycles, is 

 represented on the next page. 



Usually the different cycles which constitute the crown of tentacles are closely arranged in 

 a narrow marginal zone, so that basally the members of one cycle partly embrace those of the 

 next. 0. radiata, just mentioned, i- again exceptional in that a wide discal interspace separate- 

 one cycle from the next, the tentacular crown being unusually broad, encroaching upon the 

 peri-tome. The same feature is also characteristic of the polyps of Siderastrsea and Agaricia; 



