88 



INVERTEBRATE MORPHOLOGY. 



ous stout spines occur. Some of the hydranths are typical 

 trophopolyps (Fig. 44, tp) with filiform tentacles and a mouth, 

 but among them are gonopolyps {gp) with short-knobbed 

 tentacles below which the medusa-buds arise. In addition 

 there are also towards the periphery of the colony much 

 longer slender hydranths without a mouth and like the gono- 



FiG. 44. — Portion of Colony of Mydractinia echinata (adapted from figure by 



HiNCKSI. 



gp = gonopolyp. mp = offensive polyp. tp = trophopolyp. 



polyps in having short-knobbed tentacles, but differing from 

 them in their greater length and in never producing medusa- 

 buds. Finally, in a European species of the genus a fourth 

 exceedingly long polyp {mp), destitute of both mouth and 

 tentacles, but furnished at its free end with numerous nemato- 

 cysts, . occurs. These third and fourth varieties of polyp 

 probably are offensive and defensive in function, procuring 

 food for the colony and warding off some predatory enemies. 

 The AnthomedussB, as the medusa forms are termed, have 

 much deeper bells than the Leptomedusse, and differ from 

 the latter in that the sense-organs are light-percipient in their 



