THE CYSTIDEA 



53 



of columnals. This family includes some of the Cystidea earliest 

 studied. EchinospJiaera and Sphaeronis (p. 71) are found in the rocks 

 around the Baltic as round balls filled with radiating crystals of calcite. 

 Hence they were known to the older Scandinavian naturalists as "crystal 

 apples," arid were, as such, included by Linnaeus in his Mineral Kingdom, 

 under the name Aetites. T,heir animal nature was first demonstrated 

 by the youthful Gyllenhal (1772) in an admirable paper. He further 

 recognised, not merely their echinoderm affinities, but also that essential 

 difference between the tests of the two forms which was emphasised by 

 Joh. Miiller eighty-two years later, served as the basis of Zittel's classifica- 

 tion, and is still regarded as an ordinal character. So little were these 

 forms understood that Kb'nig (1825) placed Echinosphaera aurantium near 

 the Ascidian, Boltenia, under the name Leucopliihalmus Strangwaysi ; while 

 in 1845 M'Coy compared a Sphaeronis to the Ascidian, Chelyosoma. These 



Br' 



Echinosphaera aurantium, after Vol- 

 borth. The lettering is explained in the 

 adjoining text. Nat. size. 



FIG. XV. 



Peristomial areas of Echinosphaera 

 aurantium, showing variation in origin of 

 brachioles (Br). (After Volborth.) En- 

 larged. 



comparisons, though based on superficial similarity, with bearing on the 

 supposed relationship between Echinoderma and Enteropneusta, have again 

 been brought forward by Haeckel (1896). Genera Echinosphaera, Wahlen- 

 berg(1818 ; synn. Orystallocystis, Citrocystis, Trinemacystis, Haeckel), Ordo- 

 vician, Europe, type Echinus aurantium, Gyll. (Fig. XIV.). The smooth, 

 spheroidal theca is composed of some hundreds of irregular plates, mostly 

 hexagonal. At the aboral pole the plates, arranged in one or two fairly 

 regular circlets, form a slight projection (Sf), by which probably the theca 

 was attached, but there was no definite stem. At the oral pole is another 

 projection (0), very variable in size and shape ; this, as shown by Volborth 

 (1846), supports arms. From the figures published by Volborth (1846), 

 Miiller (1854), Quenstedt (1876), Angelin (1878), and Haeckel (1896), it 

 appears that the plates forming the oral projection, as well as the arms them- 

 selves, vary in number and position (Fig. XV.). The primitive number of 

 arms appears to be three, one anterior, i.e. opposite the anus, and two 

 lateral. The two lateral may fork, thus producing five branches in all 



