SPONGES. 



69 



StTB-OKDEK II. — LlTHISTIN^. 



This group is composed of fossil forms in which the skeleton is made up of rather 

 in-egular star-shaped radiating bodies, firmly united. Thus, a very strong and solid 

 skeleton was constructed, which has consequently been 

 well preserved in the roCks, The normal form is a mass 

 with numerous cloacal apertures of average size on the 

 upper surface, but forms quite as often grow in vase-shapes, 

 with the cloacal apertures on the inside, or like a pear, with 

 the apertures on top. The large opening in the vase-shaped 

 forms is usually described as a cloaca, though, as we have 

 seen, it is not so Ln reality. The type appears in the genus 

 Aulacopium of the Silurian, though Zittel thinks that some 

 of the Cambrian forms may belong here. 



Sub-Okdee III. — Hbxactinellin^. 



The glass sponges are remarkable for possessing six- 

 armed spicules. Two of the arms may be almost indefin- 

 itely lengthened and bound together with others in threads 

 closely resembling spun glass. In others they may be 

 shortened and split into the semblance of flowers with 

 narrow petals. The glass sponges remind the observer of 

 the calcareous sponges, but the resemblance is merely 

 superficial, and not so important as it at first appears. 

 Though the JEuplectdla is hollow and has apertures 

 through the wall as do the Calcispongise, they do not 

 lead into radiating canals, but into areolar tissue and com- 

 municate with the ampullae by means of numerous aper- 

 tures in the walls of the sacs. The outlets of the sacs are 

 large and open internally into the tube. The external and 

 internal walls are supported by the interlacing arms of the 

 crosses or hilts of the spicules, and as these are arranged 

 with great regularity, the surface of the skeleton is divided 

 into squares. The pores of the outer surface are usually 

 situated one in each of the quadrangular intervals, and the 

 clbaca occupy a similar position on the inner wall. The 

 top of the sponge is closed with a network of threads, 

 between which occur, as in Myalonema, the true cloacal 

 outlets. In fact Euplectella may be regarded as a hollow 

 Hyalonema. 



Syalonema was at first known only by the stem which 

 was highly prized as an ornament. The natives were in the habit of cleaning off the 

 sponge body from the upper part of the stem, and then reversing it in a suitable 

 standard. It was sold to strangers as the skeleton of the parasitic polyps {Palythoa) 

 which live habitually on the stem. Scientific men were at first deceived, and the true 

 character was not discovered until 1860, when Max Schultze found the sponge tissues, 



Fig. 59. — Hyalonema^ glass-rope 

 sponge. The stems are covered 

 with parasitic polyps. a,polyps 

 enlarged; b, perfect sponge. 



