REPORT ON THE HEXACTINELLIDA. 21 



oomplicated folding of the clinmber layer as occurs in the very thick-walled species 

 Pheronema (Pis. XLII.-XLVI.) and Poliopogon (Pis. XLVII.-L.). The efferent canals do 

 not, however, by any means, always remain free from the trabecular framework which is 

 so abundantly developed in the afferent spaces. This is very frequently illustrated, as, 

 for example, in Malacosaccus (Pis. XVIIL, XIX.), Crateromorpha (Pis. LXI.-LXIII.), 

 Hyalonema (Pis. XXVII.-XLJ.), Pohjloplms (PI. LIV.), Rossella (PL LV.), and others, 

 where a reticular lining penetrates from the subgastral trabecular space into the outer- 

 most diverticulum of the efferent canal system, without, however, at any time crossing 

 the lumen of the efferent passages, or entering the cavity of the chambers. The gastral 

 membrane extends smoothly over all the (excurrent) openings of the efferent canals, 

 in the form of a sieve-like net, and thus forms the boundary of a simple gastral space, 

 as in Rossella (PI. LV.), Pheronema (Pis. XLII.-XLVI.), &c. ; or it lines the niche- 

 like depressions of the wall which may be of simple or complex form, as in Mala- 

 cosaccus (Pis. XVIIL, XIX.); or finally it passes through the wide excurrent openings 

 of greatly branched efferent canals which open directly into the gastral space, and 

 extends within them on to the terminal branches, as in Hyalonema (Stylocalyx) 

 depressum (PI. XXXVI. fig. 1) and others. 



Further variations of the simple saccular form are exhibited by many Hexactinellida 

 in the formation of a terminal sieve plate covering the wide opening at the extremity 

 {Euplectella, Pis. I., V., Holascus, Pis. XV.-XVIL, Hyalonema sieboldii, PI. XXVIL, 

 and others). In many cases, too, the body -wall exhibits a more or less regulai-ly arranged 

 set of gaps, by means of which a direct communication is established between the gastral 

 cavity and the external medium. While these gaps in the walls of Euplectella 

 (PI. I.) and Taegeria (PL VII.) are circular, and bounded by an iris-like membrane' 

 which is capable of contraction, in Walteria (PL IX.) they occur as irregularly 

 angular meshes, like the lattice-work of a basket. Striking modifications may arise 

 by the formation of a stalk, which is, indeed, always in the form of a simple continua- 

 tion of the lower portion of the body-wall, from which, however, it often differs 

 widely in diameter. It varies greatly in the length attained, and is more or less 

 sharply truncated. The stalk is generally quite round and smooth, but frequently with 

 characteristic curvature [Gaulophacus elegans, PL XXV.), or prominent protuberances 

 [Crateromorpha murrayi, PL LXIIL), and is sometimes even branched {Symjxtgella mix, 

 PL XXII. fig. 4). It is either solid or tubular, generally the latter in long stalks (Canlo- 

 pthacus), and its lumen either opens into the gastral cavity, or is in communication with, 

 and so belongs to the efferent canal system. By the expansion of the upper oscular 

 margin many species, e.g., Rhahdocalyptus mollis (PL LXIV. fig. 1) and others, acquire 

 a funnel-like shape. A further widening and flattening leads to the formation of a flat 

 saucer-like body, while a more unilateral growth results in an ear or shell-like form 

 {Euryplegma auricidaris, PL OIL), or even in certain circumstances in a simple perpen- 



