TYPE GCELBNTERA. 69 



termined, and iu the Sponges the form may become so irregu- 

 lar that they may be considered to be destitute of axes. In 

 such forms as the Medusae, however, a typical radiate form 

 occurs, there being two or more similar axes at right angles 

 to the vertical one, and throughout the higher members of 

 the group this radiate symmetry is more or less apparent, 

 though it becomes decidedly obscured in certain Anthozoa by 

 a pronounced tendency towards bilaterality, which in a few 

 forms {Cerianthidce) actually replaces it. 



In correspondence with their low grade of general struc- 

 ture there is no very extensive differentiation of tissues. A 

 considerable degree of division of labor of course occurs 

 among the cells, and cells having the same function may be 

 aggregated together so as to form a somewhat definite tissue, 

 as in the case of the nerve, muscle, and reproductive cells, but 

 even in these tissues there seems to be a considerable amount 

 of individuality retained by the constituent cells, and the tis- 

 sues can only be regarded as exceedingly diffuse. Of organs, 

 except in some colonial forms with division of labor among 

 the constituent individuals, it is hardly correct to speak, the 

 Coelenterates not having progressed beyond the organ stage 

 of individuality. 



The type Coelentera may be divided into two subtypes, 

 the Porifera, or Sponges, and the Cnidaria. 



I. Subtype Porifera. 



The Sponges, on account of their iixed life and irregular 

 form, were long regarded as plants, and it is only within com- 

 paratively recent times that their true relationships have been 

 ascertained. They are almost exclusively marine in habitat, 

 occurring in large numbers in the warmer seas, and inhabit 

 the ocean depths as well as the shallower waters. A few 

 genera, e.g. Spongilla, Ephyatia, represented by numerous 

 species, are inhabitants of fresh water. 



The simplest Sponges (Fig. 34) have the form of a hollow 

 cylinder fixed at one end, while at the other is an opening, 

 the osculum, and scattered over the surface of the cylinder 

 are a number of smaller openings, the pores. Through these 



