6 SPONGES 



surface of the body wall. In the first place, such increase may take 

 place more or less evenly and regularly in all parts, hut more 

 rapidly in one direction than in another ; then the sponge person 

 becomes an elongated cylinder or tube if the growth be chiefly 



Yonng specimen of CluthriiM lUii^alum, O.S., wrth one osculum, x6. 



vertical, or assumes the form of a cup or saucer if the growth be 

 chiefly horizontal. In the second place, the growth may be uneven 

 or irregular, being more rapid in one part of the sponge body than 

 in another, or taking place chiefly in certain limited' regions. In 



Fig. 7. 



Clathrina clathrus, O.S., natural size, scmi-diaKraniniatic combinpd figure. On tlie left the 

 spongfi is rcpresiMited in the state of extreme expansion, jiassing gnwlually into that of extreme 

 contraction on tlie right. osc, osculum ; cl.osc, closed osciilum ; contrAvfc, elevated "conules" 

 in the contractc^d jwrtion representing oscula contracted ; sph, sphincter of osculum ; div, 

 diverticula ; osc.div, vertically directed diverticula from which new oscula arise. 



such cases either the body wall must be thrown into folds, or the 

 primitive form of a vase or sac will be distorted or modified in 

 various ways. 



Instances of both tubular and cup-sliaped sponge individuals are 

 common amongst the Hexactinellids. Tlie lirst type is well seen in such 

 forms as Regudrella (Fig. 1 8) and EiqAcctella (Fig. 1 6) ; the second in such 

 a form as Asconcma (Fig. 17). The tubular forms may assume an erect 



