193 



PORIFERA 



... 



Hexactinellids contrast with most other sponges by their 

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

 the Calcarea which inhabit shallow water exhibit almost as 

 perfect a symmetry. 



The structure 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 

 dm. tissue, between a der- 

 mal membrane on the 

 outside and a similar 

 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 



sd. tr. 



Longitudinal section of a young specimen of 



Lanuginella pupa O.S., with commencing formation trabecular Space ; Oil 



of the oscular area, x 35. d.m, Dermal membrane ; l Pav j no . fn P phnrnW 



g.m, gastral membrane ; pg, paragaster ; sd.tr, sub- | eavm g e CUamuerS 



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



uze.) 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 



I 



