156 Prof. W. J. Sollas on the 



contraction. It is clear from the preponderance of tlic con- 

 centric fibres that the chief work done is in the contraction 

 and closure of the sphincter ; its return, on the relaxation of 

 the concentric fibres, to a completely open state is completed 

 by the opposing radiate fibres. This sphincter (PI. VII. 

 fig. 6) is precisely homologous with the chonal sphincters of 

 Geodina and Stellettina. The fibrous layer with its spicules 

 is homologous with the fibrous globate layer of the Geodina. 



The Mark. — The mesoderm of the mark chiefly consists 

 of a clear transparent matrix densely charged with more 

 refractive, transparent, minute round granules (Tu-ffuiJ to 

 TT^lna iiich in diameter), so thickly strewn that they appear 

 almost in contact, and so evenly that no place seems to 

 contain more than another : here and there at intervals 

 small nuclei appear ; and the mark surrounding them takes a 

 deeper stain with carmine than elsewhere. If such a tissue 

 as syncytium exists anywhere, then surely it is here. 

 Fusiform fibres occur in the vicinity of the spicule-fibres, 

 running parallel to the spicules, and also about the large 

 water-canals, building around their course an indefinite fibrous 

 layer. Most markworthy arc certain large granular amoebi- 

 form cells with characteristically large oval nuclei and round 

 nucleoli, like amoebiform ova, which are found embracing the 

 spicules (PL VI. fig. 17) as though they were wandering cells 

 creeping along them. 



The endodermic cells of the flagellated chambers contribute 

 a large share to the substance of the mark ; they now appear 

 as spherical vesicles 377^,0 inch in diameter, containing a 

 spherical nucleus ^ ^ ^ ,-, ^ inch in diameter, which colours in- 

 tensely with carmine. Carter has represented one of these 

 chambers in his description of Tetilla (Ann. & Mag. Nat. 

 Hist. 1872, vol. ix. pi. xxii. fig. 7), but has mistaken it for an 

 ovum. 



The Ova. — Sparsely scattered without apparent order 

 through the mark are a number of very variously-shaped 

 A7noeha-\ike cells, distinguished Ify their disproportionately 

 large nucleus and nucleolus. They occur of all sizes, from 

 y^o^ inch diameter to t-ju inch, and are without doubt the 

 ova in various stages of development. As soon as they attain 

 a size of about -j— inch across they occupy a distinct cavity 

 in the mark, which serves as a brood-chamber. Pseudopo- 

 dial extensions, which may become branched, proceed from 

 them, and, passing out of the brood-chamber, wander for a 

 considerable distance on the surrounding substance of the mark. 

 No trace of fibrillation could be detected in these processes. 

 The large oval nucleus, sometimes -^^^ inch in diameter, with 



