76 INVMRTEBUATE MORPHOLOGT. 



dividual cannot withstand. They are spherical bodies con- 

 sisting of a mass of cells richly laden with food-matter, and 

 enclosed in a double chitinous wall, with an opening at one 

 point, a niimber of siliceous spicules, in the allied genus 

 Aphydatia of a very characteristic form and known as amphi- 

 discs, being arranged between the two layers of the wall. On 

 the approach of cold weather the Sponge dies down and the 

 gemmules thus fall to the bottom of the ponds or streams, 

 where they remain unchanged until the approach of warmer 

 weather, when the internal cellular mass flows out through 

 the pore (which is closed only by a thin membrane) and de- 

 velops into a new Spongilla. 



The relationships of the Sponges have long been a matter of discussion. 

 For a long time they were regarded as plants and later as colonies of .Pro- 

 tozoa, but the discovery of sexual reproduction in them and of their mode 

 of development demonstrated that they were to be considered Metazoa. 

 At present the question as to whether they are to be associated with the 

 Cnidaria among the Coelenterates or regarded as a distinct type is still 

 open, though the weight of evidence and authority is in favor of their 

 Coelenterate character. Such simple forms as Leucosolenia certainly point 

 in that direction, and, if the occurrence of a sterrula formed by immigra- 

 tion prove the typical mode of development, the embryology of the Sponges 

 presents stages up to the formation of the ciliated chambers which are 

 step by step comparable to what occurs in the Cnidaria. 



II. Subtype Cnidaria. 



The Cnidaria, like the Sponges, have in their simplest 

 forms the general form of a hollow cylinder open at one end 

 and consisting of but two cellular layers, the ectoderm and 

 endoderm, between which is interposed a fibrous or gelatinous 

 mesogloea which may or may not contain cells. Differences 

 from the Sponges are found in the occurrence, except in one 

 or two forms, of a number of elongated, contractile processes 

 or tentacles around the mouth of the cylinder (see Fig. 33), 

 and in the absence of inhalent pores upon its surface. Such 

 simple forms are known as polyps, and they are usually 

 attached organisms with little or no power of locomotion. A 

 large number of Cnidaria present a very different form, how- 

 ever, being disk- or bell-shaped, a process comparable to the 

 clapper of a bell hanging down from the centre and hav- 



