90 



INVERTEBRATE MORPHOLOGY. 



knobbed ; these are probably offensive polyps, of use to the 

 colony in obtaining food, and are known as dactylozoids (Fig. 

 46, d). The cavities in the corallum in which the gasterozoids 

 live are divided by transverse partitions into chambers into 

 the outermost of which the hydranth may be retracted, the 

 arrangement recalling what occurs in the corallum of the fossil 

 Tabulate Corals. In the genus Stylaster, however, which 

 forms a rose-red branching corallum, these partitions are 



<1 



rr 



FIG. 46. PORTION OF COLONY OF Millepora (after MOSELEY). 

 co = corallum. d = dactylozoid. g = gasterozoid. 



wanting, a calcareous cone, the columella, projecting upwards 

 from the floor of the cavity. 



In one species at least of Millepora a well-marked and typi- 

 cal alternation of generations occurs, medusae being formed 

 which are set free and develop the reproductive elements in 

 the ectoderm of the maimbriuin. In the majority of the 

 members of the order, however, the medusa-buds are never 

 set free, and are usually much degenerated, and indeed in 

 Millepora alcicornis they may be said to have completely dis- 

 appeared, and with them the alternation of generations. The 

 medusa-buds develop on the walls of the coenosarcal tubes, 

 and lie in cavities in the corallum which open to the exterior 

 by a pore through which the egg-embryos escape. 



