72 SYNOPTIC COLLECTION. 



(fig, 14), which is filled up with granular cells. Between 

 the two layers of cells there is a narrow bright zone which 

 Schultze considers the first indication of the gelatinous 

 inter-layer, mesoderm, that reaches such a great develop- 

 ment in most sponges. 



The spicules appear in the mesoderm, first in the form 

 of slender, straight little rods pointed at both ends (see 

 fig. 15), which fact favors the view that the first formed 

 spicules were one rayed and straight. They thicken as 

 they grow and curve into a slightly S-shaped form. After- 

 wards three rayed and four rayed spicules are formed, one 

 of the arms of which extends inward, while the other 

 three are on the surface and probably serve for protec- 

 tion. 



The larva lengthens, pores form in the wall, and the 

 large opening at the top breaks, through the ectoderm. 

 It is now clear that this opening is not a mouth nor a 

 primitive character, but a secondary feature, occurring in 

 a past embryonic stage, and is in reality a cloacal open- 

 ing for the ejection of the waste products of the body. 

 The cells of the endoderm acquire collars and flagella. 

 The body cavity of the young Sycandra is now a simple 

 ampulla having as yet no branches. In this stage it is 

 identical with an adult Ascon, like Ascetta, which it 

 structurally represents. Later the mesoderm thickens, 

 the pores grow into tubes, the ampullaceous sacs are 

 formed near the food supply, the cells of the body cavity 

 lose their flagella, the cells of the ampullaceous sacs 

 acquire collars and flagella, and from that time the work 

 of taking food and digesting it for the use of the other 

 cells is done by them. Thus the single primitive diges- 

 tive cavity becomes a cloacal trunk, pores become tubes 

 branching from this trunk, and the function of the cavity 

 is transferred to the little sacs or ampullae formed in the 

 canals as they are stretched out by the thickening of the 

 mesoderm. This is a process of reduction resulting in 

 transforming a normally formed, symmetric, vase-shaped, 



