1554 THE VOYAGE OF H.M.S. CHALLENGER. 



Subfamily 3. Dictyochida, Haeckel. 



Definition. — C annorrhaphida with a skeleton composed of numerous annular 

 pieces, which lie tangentially scattered on the surface of the calymma. Each piece is 

 either a simple hoUow ring or a pileate and reticulate cap, composed of a ring and 

 several connected bars. 



Genus 661. Mesocena,^ Ehrenberg, 1841, Abhandl. d. k. Akad. d. 



Wiss. Berlin, p. 401. 



Definition. — C annorrhaphida A;v-ith a skeleton composed of sim^ile annular 

 pieces, each of which is a circular, elliptical or polygonal, not fenestrated, ring, with 

 or without radial spines. 



The genus Mesocena is the simplest form of the Dictyochida, and no doubt the 

 common ancestral form of this subfamily. The siliceous pieces, which are scattered in 

 variable and indefinite number in the calymma, are simple hollow rings, with or with- 

 out spines on the periphery. Ehrenberg, who first described and figured such rings 

 (found fossil in different Tertiary rocks) has mistaken them for Diatoms. Some 

 species, which I found in the Challenger preparations, leave no doubt that these 

 rings are the siliceous pieces of the skeleton of the simplest Dictyochida. They are 

 scattered in great numbers in the spherical calymma, which surrounds a tripylean 

 central capsule with all the characters of the Ph.^odaria. In the living body the rings 

 probably always lie in the spherical periphery of the extracapsular jelly-veil, in 

 tangential planes, whilst in some of the Challenger preparations the rings were 

 scattered in hundreds thi'oughout the whole jelly -mass. In a few species the rings 

 are quite simple, circular or elliptical, smooth, and without teeth or spines. In the 

 majority of species some teeth or radial spines, regularly disposed, arise from the 

 periphery of each ring (two, three, four to eight ; sometimes sixteen, eight smaller 

 alternating with eight larger spines). In some species small teeth occur on the 

 inner margin of the rings. The number of radial spines seems to be rather constant 

 in all the rings of one and the same individual, ■with the exception of a few variations. 

 Thus in Mesocena octogona I found here and there single rings with seven or nine 

 teeth, instead of the usual number eight. Ehrenberg enumerated sixteen different 

 species of Mesocena; many, however, of these are synonymous, being founded on slight 

 variations in the number of the teeth ; of others he has given only the name, but neither 

 a figure nor a description {e.g., Mesocena stephanolithis, Mesocena spongolithis, &c.). 



' ilfesocewa = Hollow in the centre, annular; ^«aov, xei/o'j. 



