SPONGES 7 



cylindrical form (Euplectella suberea\ or may be more or less curved like 

 a cornucopia (E. aspergillum). A remarkable instance of horizontal growth 

 of the body wall is seen in Caulophacus (Fig. 20, (7), where the wall of the 

 gastral cavity is turned outwards and downwards, and the sponge being at 

 the same time stalked, a form like a mushroom results, in which the upper 

 convex surface of the disc represents morphologically the inner surface of 

 the body, and the rim of the disc is the down-turned margin of the 

 osculum. An approach to this condition is seen in the fossil Ventriculites. 

 Some specimens have the body shaped like a paper basket, while others 

 have the margin very much expanded and everted (Fig. 23). Man tell 

 (1822) makes the suggestion that the differences in different specimens of 

 Ventriculites may be due to contractility. 



A good example, on the other hand, of the effect of rapid local growth 

 is seen in the Hexactinellid sponge Euryplegma (Fig. 20, .4). Here the 

 primitive vase-like organism grows with great rapidity on one side, and 

 scarcely at all on the other. The result is an ear-like or shell-like form, 

 in which the concave side represents the gastral cavity, and the whole free 

 edge the margin of the osculum (ra.osc). This mode of growth is carried to 

 its extreme in Poliopogon (Fig. 20, B\ where the sponge has become simply 

 a curved plate, of which the upper edge represents the oscular margin 

 (ra.osc), the concave side the gastral cavity. 



If the portions of the body wall which grow more rapidly are 

 distributed, so to speak, in patches, the result will be the formation 

 of diverticula or folds. The best instances of this are seen in the 

 calcareous sponges, all of which begin their existence as a vase-like 

 organism of very primitive structure, termed the Olynthus (Figs. 1 

 and 2, a). Hence the Calcarea are specially suited for tracing out 

 the processes of growth by which the often complicated body form 

 is attained. 



In the most primitive Calcarea, the Ascons, the Olynthus grows 

 in height, becoming tubular, while at various points on the surface 

 hollow diverticula are thrown out on every side. The diverticula 

 increase rapidly in length, and become branched, and by coalescence 

 and anastomosis of the branches there arises a network of tubes, 

 which surround, and open into, the central oscular tube, represent- 

 ing the original Olynthus. The continuous cavity which extends 

 through the whole tubar system is, of course, the now greatly 

 ramified and subdivided gastral cavity. 



Two types of body form can be distinguished in Ascons as the result 

 of simple variations in the mode of growth above described. In the 

 family Clathrinidae the vertical growth of the Olynthus is comparatively 

 slow, the horizontal growth of the diverticula comparatively rapid. In 

 the family Leucosoleniidae, on the contrary, the Olynthus grows rapidly 

 in height, while the diverticula, though more numerous, remain relatively 

 small. Hence the typical Ascon person is, in the genus Clathrina, a 

 dense network of ramifying tubes opening by a short and often in- 

 significant oscular tube (Figs. 2, 6, 7, 8 ; cf. Fig. 65, A), and in the genus 



