198 



PORIFERA 



Hexactinellids contrast with most other sponges by tlieir 

 symmetry. It should not, however, be forgotten that many of 

 tlie Calcarea which inhabit shallow water exliibit almost as 

 perfect a symmetry. 



The strticture of the body-wall in Hexactinellida is so constant 

 as to make it possible to give a general description applicable to 



all members of the 

 group. It is of con- 

 siderable thickness, 

 but a large part is 

 occupied by empty 

 spaces, for the actual 

 tissue is present in 

 minimum quantity. 

 In the wall the cham- 

 ber-layer is suspended 

 by trabeculae of soft 

 ■d.m. tissue, between a der- 

 mal membrane on the 

 outside and a similar 



scl. tr 



gastral membrane on 

 the inner side (Fig. 

 89). Thus the water 

 entering the chambers 

 through their numer- 

 ous pores has first 

 passed through the 

 ostia in the dermal 

 membrane and tra- 

 versed the subdermal 



Fig. 89. — Longitudinal .section of a yonng specimen of 



iffl)iM(?meZfai)«2)n O.S., with commencing formation trabecular Space : OU 



of the oscular area, x 35. rf.?)», Dermal membrane ; ■]„„,.; j.i i^ i 



g.m, gastral membrane ; pg, paragaster ; sd.tr, sub- ^^^^^^g '''''^ cnamDSrs 



dermal trabeculae ; Sg.tr, subgastral trabeculae. it flows thrOUeh the 

 (AfterP. E. Schulze.) , . , . ? , 



subgastral trabecular 

 space and the ostia in the gastral membrane, to enter the para- 

 gaster and leave the body at the osculum. The trabeculae 

 and the dermal and gastral membranes together constitute the 

 dermal layer. This conclusion is based on comparison with 

 adults of the other groups, for in the absence of embryo- 

 logical knowledge no direct evidence is available. According to 



