386 



GENERAL ZOOLOGY 



ings of the main cavity, called flagellated chambers or ampullae, 

 which alone contain choanocytes. Water enters the ampullae 

 through pores and passes into the central cavity, now called the 

 cloacal cavity, from which it leaves by the osculum (Fig. 217B). 

 The Leucon type of sponge (Fig. 217C) results from a separation 

 of the ampullae from both external and cloacal surfaces by an 

 increase of mesodermal tissue, the ampullae retaining their 

 connections with both surfaces by means of narrow canals. 

 Both the incurrent canals (the original pores), leading to the 

 ampullae, and the excurrent canals, leading away from them to 

 the cloacal cavity, may be enlarged to form subdermal and sub- 



A B c 



Fig. 217. — Diagrams of three structural types of sponges. A, Ascon type; B, 

 Sycon type; C, Leucon type. The dermal epithelium is indicated in light line, 

 the gastral epithelium in heavy black. Since the outer layer of the sponge larva 

 becomes the inner layer of the adult, there is some difficulty in applying the 

 terms ectoderm and endoderm. As it is, the dermal epithelium corresponds in 

 position to the ectoderm of other forms and the gastral epithelium to the endo- 

 derm. c, cloacal cavity; f, flagellated chamber; g, gastral cavity; o, osculum; 

 p, incurrent pores; s, subdermal cavity. 



cloacal spaces. In any case the choanocytes are confined to the 

 ampullae. 



The outer layer of the body of the sponge is composed of a 

 single layer of flattened cells. The gastral cavity and the 

 ampullae are lined with choanocytes. The remaining cavities 

 are lined with a smooth epithelium. The middle layer, or meso- 

 derm, lying between the dermal layer and the gastral layer or the 

 derivatives of the latter, varies in thickness and contains a variety 

 of structures, some of which are skeletal parts, such as the hard 

 calcareous or siliceous spicules, or the softer horny fibers of 

 spongin. Both spicules and spongin are present in many sponges, 

 though some contain neither. Some spicules are narrow rods, 

 pointed or rounded at the ends, others are tri- and tetra-actinal 



