HYDROCORALLINES. 



35 



Fig. 56. Hydractinia (Re- 

 cent), showing 3 types of polyps 

 arising from spinous hydrophy- 

 ton (enlarged). 



tinia, an aberrant type of Hydroid. This latter will serve to 

 elucidate the structure of the Stromatoporoids. 



Hydractinia (Fig. 56) commonly grows as an incrustation on the 

 shells of dead gastropods, which have been seized as a habitation 



by hermit crabs. It is also found, how- 

 ever, encrusting rocks. The polyps are 

 naked, i. e. y not protected by hydrothe- 

 cae, and of several kinds. The ordinary 

 feeding polyp or nutritive zooid has a 

 mouth and tentacles. A second type of 

 polyps has short, club-shaped tentacles 

 and bears a cluster of reproductive buds 

 or gonangia. These are the gonopolyps. 

 A third and sometimes a fourth order of 

 polyps occur, scattered about or confined 

 to certain regions. These have an offens- 

 ive and defensive function and apparently 

 warn the colony of approach of danger. 

 The colony secretes a basal horny or calcareous structure, the 

 hydrophyton, which consists of successive, slightly separated, hori- 

 zontal laminae, supported by numerous vertical columns known as 

 radial pillars (Fig. 57). From the surface 

 arise hollow spines and tubercles, which 

 serve as a protection when the polyps are con- 

 tracted below their summits. The spaces be- 

 tween the laminae communicate with the sur- FlG Hydractinia 



face by means of tubes, and branching Cross-section of hydrophy- 



grooves or aslrorhizcz are commonly present ton through a spine, show- 

 on the surface. In Millepom the colony Se- ing horizontal lamin* and 



radial pillars. 



cretes a calcareous structure {coenosteiun) not 



unlike some coral masses. This consists of a network of calcare- 

 ous fibers and is traversed at frequent intervals by large tubes sur- 

 rounded by a ring of smaller ones. The larger tubes (gastropores) 

 lodge feeding polyps, and the smaller (dactylopores) lodge fighting 

 polyps. Both sets of tubes are divided in the older part by hori- 

 zontal partitions or tabulae. 



The extinct Stromatoporoids combine the characteristics of the 

 two types outlined above. They consist of hemispherical, spread- 

 ing, encrusting, columnar, or nearly globular calcareous structures 

 sometimes attaining a diameter of five feet or over, or a length of 



