SPONGES, ZOOPHYTES, AND SEA-FIRS. 



57 



substance of the branches as to be discernible only with 

 difficulty. They are arranged in two rows. 



The last family of the Calyptoblastea is the Plumularidae, 

 including some of the most delicately beautiful of the 

 zoophytes. Most are beautifully and elaborately branched, 

 so as to produce "plumes" rivalling those of the most 

 delicate ferns (see Fig. 9, p. 29). In all the cups are 

 sunk into the branches, and are placed on one side of the 

 branches only. As they are usually small, one result of 

 this arrangement is to make them very inconspicuous, so 

 that to the unaided eye there is nothing to destroy the 

 plant-illusion. The gonothecae always contain fixed sporo- 

 sacs. A final peculiar character is that the colony bears, 

 in addition to its zooid-cups, much smaller cups, called 

 nematophores, which contain stinging- cells. These are 

 usually very minute, and require the aid of the micro- 

 scope before they can be seen. Of the Plumularians 

 we shall describe one example only, the delicate little 

 Plumularia setacea, which is quite common on the shore 

 rocks. Its graceful plumes reach a height of over an inch, 

 which is not small for a littoral zoophyte, but their texture 

 is so delicate and fragile 

 that they are not easily 

 seen. Each tiny plume 

 rises independently from 

 the creeping stolon, and 

 is so transparent that, 

 except when the white 

 reproductive capsules are 

 present, it requires a 

 quick eye to discern it. 

 The special peculiarities 

 of the species lie in the 

 minute structure of the 

 pinnas, or branches. 

 Examined with the 

 microscope these will be 

 found to consist of joints 

 which are alternately 

 long (d) and short (e), 

 and of which the longer 



FIG. 22. Magnified branch of Plumularia. 

 The letters are explained in the text. 



