VERRILL, SYNOPSIS OP POLYPS AND CORALS. 17 



HI. Synopsis of the Polyps and Corals of the North Pa- 

 cific Exploring Expedition, under Commodore C. Ring- 

 gold and Captain John Rodgers, U. S. N., from 1853 

 to 1856. Collected by Dr. Wm. Stimpson, Naturalist to 

 the Expedition. With Descriptions of some additional Spe- 

 cies from the West Coast of North America. 



BY A- E. VERRILL. 



PART in. MADREPORARIA. With two Plates. 

 [Communicated February 29, 1865.] 



In a natural arrangement of Polyps, as stated in 

 Part I. of this series, the order ACTINARIA should follow the 

 ALCYONARIA, they standing next to them in grade, as is 

 shown by many features of their structure. Thus the Ac- 

 tinidae have a highly specialized muscular basal region, which 

 is used in locomotion in various forms ; they have a more 

 highly developed tentacular system than the Madreporaria, 

 these organs often being variously branched or lobedj and 

 frequently there are various other ambulacral organs, either 

 within the tentacles, around the mouth, or on the sides be- 

 low the tentacles, such as lobed or foliaceous appendages 

 for respiration or adhesion, colored spherules, perhaps organs 

 of visual sense, suckers, cinclidae, pores, tubercles, &c., 

 nearly all of which are wanting in the present order. The 

 walls also are highly muscular throughout, while in MADRE- 

 PORARIA the lower portion becomes solidified by calcareous 

 deposits and serves merely for attachment or protection. 



Again, if we take into consideration the number or 

 repetition of the lateral homologous elements or spheromeres 

 around the longitudinal axis, or rather on the two sides of a 

 median plane, we find that in each order there are genera 

 having a definite and small number, which in Madrepora is 

 twelve, as in Peachia and other Actinia, while other gen- 

 era in each have a large number, increasing indefinitely (as 

 Fungia in MADREPORARIA, Metridium, &c. in ACTINARIA). 

 While, therefore, this character can be used only to indicate 

 grade within each order, it is nevertheless true that the AC- 

 TINARIA present some genera in which the number is six, the 



COMMUNICATIONS ESSEX INST., VOL. V. 3. MAT 12, 1866. 



