44 



THE CYSTIDEA 



primitive class of Echinoderma, comparable to the Pentactula 

 stage passed through in the development of all their descendants. 

 The more characteristic and imdoubted Amphoridea, however, 

 represent only a stage in the development of the Echinoderm type 

 and not a divergence ; they are too intimately connected with 

 more specialised Cystidea to warrant separation as a class. It 

 should also be remembered that, though such a stage as this 

 probably was actually passed through, still forms are liable to be 

 referred here, owing to our ignorance of their true structure. 



FAMILY 1. ARISTOCYSTIDAE. Amphoridea without extension of food- 

 grooves, epithecally, endothecally, or on exothecal skeletal processes. 

 Theca composed of numerous plates without regular arrangement or 

 specialised structure. No stem. Genera Aristocystis, Barrande (1887), 

 Ordovician, Bohemia (Fig. II.), is in many respects the simplest Echino- 

 derm known. The ovoid 

 theca is composed of 150- 

 200 plates, of no definite 

 shape or arrangement, but 

 with a tendency in the nar- 

 rower, aboral half of the 

 theca to form transverse rows 

 of elongate hexagons. The 

 animal usually fixed itself to 

 some solid body by a portion 

 of the theca at or near the 

 lower pole (B). At the upper 

 pole is the mouth (0), a 

 wide slit in the transversal 

 plane, with slightly raised 

 edges. About a third of the 

 way down the theca is the 

 round anal opening (As), 6-8 

 mm. in diameter, closed by 



six or seven triangular plates, meeting in the centre, and known as 

 " the valvular pyramid." Between mouth and anus, and usually a 

 little to the left, are two smaller openings a transverse slit (M) 

 close to the mouth ; and a round pore ((?) close to the anus. Of these M 

 is regarded by P. H. Carpenter (1891) as the hydropore, and G as the 

 gonopore, a view accepted by Haeckel (1896) and adopted here. There is 

 no trace of calcified arms or bracbioles, whether jointed or solid, nor even 

 of epithecal or hypothecal extensions from the hydrocoel ring or from the 

 mouth. The hydropJiores palme'es described by Barrande, and supposed by 

 Neumayr (1889) and P. H. Carpenter (1891) to be subtegminal ambulacra, 

 are really epithecal food-grooves, and have not been proved to belong to 

 this genus. The plates of the theca are thick, especially at its lower end ; 

 they are said by Barrande to be composed of three layers (Fig. III.) : 

 (e) outer, thin, smooth, and solid ; (m) middle, thick, pierced by irregular 

 canals, more or less at right angles to the outer surface ; (h) inner, thin, 



II. 



Aristocystis bohemicus. 1, side view ; 2, oral view, 

 both % nat. size ; 3, base, showing impression of Gastro- 

 pod shell, xg. The lettering is explained in the adjoin- 

 ing text. (All adapted from Barrande.) 



