PORIFERA. 163 



occurs in all sponges. The ova and sperm are developed in 

 the mesenchymatous layer. The male and female cells orig- 

 inate from the same individual (hermaphroditism} . Usually 

 however the sexes mature at different times. 



208. Development. Fertilization of the ovum and early 

 cleavage take place in the mesenchyma near the incurrent 

 canals, by means of which the spermatozoa find entrance. 

 Cleavage is total and for the most part equal (see 51), pro- 

 ducing an oval blastula which swims freely by means of cilia 

 or flagella. While there are some peculiar features about the 

 gastrulation, a gastrula or two-layered embryo is ultimately 

 formed. At this stage the embryo settles to the bottom and 

 becomes attached by the end containing the blastopore, which 

 thus becomes obliterated (Fig. 75, bl). An excurrent pore 

 breaks through at the opposite end, and the numerous incur- 

 rent pores are formed at the sides. The mesenchyma seems 

 to be formed by cells which migrate from the other layers 

 into the segmentation cavity, thus filling it. The entoderm 

 outpockets into the mesenchyma, establishing connection with 

 the ingrowing ectoderm, thus forming the incurrent canals 

 (see Fig. 76). In most species the process is more complex 

 than that described here. 



209. Classification. 



The divisions of the group Porifera are made on the basis of the dif- 

 ferences in the skeleton. Two principal classes may be recognized, as 

 follows : 



I. Calcarea. Sponges in which the skeleton is composed of calcareous 

 spicules. Laboratory type, Grantia. 



II. Non-Calcarea. Sponges with glassy (siliceous) spicules, or with 

 horny (spongin) fibres, or with merely a gelatinous mesenchyma. Labo- 

 ratory types : the fresh-water sponge ; the commercial sponge. 



210. Sponges are chiefly marine animals, and flourish in 

 all the seas and at any depth. The larger horny sponges of 

 which the bath sponge is the skeleton are found in the warmer 

 seas, and in relatively shallow water. By reason of their 

 budding and branching, the sponges form immense colonies 

 or beds, and many other forms of life associate with them in 



