8 



SPONGES 



Leucosolenia, a large and erect oscular tube giving numerous diverticula 

 of comparatively small calibre, which increase in length towards the base 

 of the tube, where they tend to branch and anastomose (Figs. 3, 4, 5 ; 

 cf. Fig. 65, B). 



A body form very similar to that of Ascons, and the result of a 

 similar mode of growth, is also of common occurrence in the order 

 Dictyonina amongst Hexactinellids (Figs. 21, 22). The primitive 

 vasiform sponge person becomes first tubular and then branched, 

 and by anastomosis of the branches a network of tubes results. 



In the higher calcareous sponges, the order Heterocoela, we find 

 a mode of growth which, though essentially similar to that found 

 amongst Ascons, leads to a body form considerably different, and 

 in most cases much simpler. As typical may be taken the genus 



Fio. 8. 



Clathrina lacunosa, 

 Johnst., colony with 

 two oscula, x 4. 



Fio. 9. 



Sycon ciliatum, Fabr. 

 X 2. 



Fio. 10. 



A, Sycon raphanus, O.S. (after Schmidt), 

 x 5. 'B, Sycon humboldtii, Risso ( = Dun- 

 fterviUia corcyrensis, O.S.), (after Schmidt), 

 X2J. 



Sycon, where the Olynthus sends out numerous breast-shaped 

 or thimble-shaped diverticula, more or less regularly disposed on 

 every side. New outgrowths continually appear just below the 

 oscular margin and continue to increase in size, but unlike what 

 has been described for Ascons, the diverticula in Sycons have a 

 limited growth. The size attained by the diverticula is greatest at 

 the sides and towards the base of the sponge. As a result of this 

 mode of growth the sponge assumes a strobiloid form, which in some 

 primitive types is more or less retained throughout life. In 

 most Sycons, however, the diverticula become united by secondary 

 growths at their apices, and are thus rendered indistinguishable in 

 an external view of the sponge. Hence the effect produced is 

 simply that of a great thickening of the body wall. The Sycons 

 furnish, in fact, a clear instance of the body wall of the primitive 



