H44 AN INTRODUCTION TO ZOOLOGY. 



is the endoderm (see p. 78). The main substance 

 of the sponge, which contains and secretes the skele- 

 ton, is composed of Amceba-like cells, forming a firm 

 and contractile flesh, or parenchyma^; this corresponds 

 with the mesoderm, or middle layer of the animal body. 

 The outside layer, or skin, called the ectoderm, con- 

 sists of a thin flat layer of epithelium cells (see p. 41). 

 The parenchyrna layer contains specialized male and 

 female cells (spermatozoa and ova) : the latter are 

 amoeboid and motile. . 



The surface of a sponge is perforated by holes, 

 whence the group receives its name of Porifera. These 

 holes are of two kinds, small and large, called respec- 

 tively pores and oscnla. Canals, lined with cells 

 derived from the ectoderm, pass from the pores into 

 the substance of the sponge, and open into small 

 chainbers. Each of these gives rise to another tube, 

 and by the union of this set of tubes a large canal is 

 -formed, which opens into an osculum. The water, 

 carrying with it food particles, enters by the pores, 

 and is exhaled through the oscula. The chambers 

 and exhalent tubes, are lined with endoderm. 



The sponge, like the vast majority of invertebrate 

 animals, has a larval stage. The larva, which is at 

 first a free ciliated form, fixes itself, and becomes a 

 simple tube with one osculum : the sponge, as we usu- 



' Literally, stuff employed in filling up (Gk. ey-^eo), to'fiU 

 up by pouring in) ; a name employed to designate the sub- 

 stance of various tissues composed of similar cells or parts. 



