TYPE COSLENTEKA. 



89 



function, whence the medusae are frequently termed ocellate, 

 and, secondly, in that the re- 

 productive organs develop in 

 the wall of the manubrium 

 instead of on the line of the 

 four radiating canals. Ten- 

 tacles as a rule occur at the 

 margin of the bell, and it is at 

 their bases that the eyes are 

 formed. Occasionally, as in 

 Margelis (Fig. 45), tentacles 

 also arise from the end of the 

 manubrium. As a rule, the 

 sexual generation is composed 

 of free-swimming medusae, as 

 in Coryne, Margelis, and Pen- 

 naria, but not infrequently the 

 medusa-buds become retarded 

 in their development, as in FlG - 45 - MEDUSA OP Margelis caroli- 

 Hydractinia, Clava, Tubularia, 



and to an extreme extent in Eudendrium, and all intermediate 

 stages between the two extremes are to be found. 



6. Order Hydrocorallinee. 



The Hydrocoralliuge are colonial marine forms represented 

 by the Stag's-horn Coral, Millepora, and characterized by the 

 densely ramified coenosarcal tubes being enclosed in a mass 

 of carbonate of lime secreted by the coanosarcal ectoderm 

 and taking the place of the perisarc. From minute pores on 

 the surface of this calcareous mass, the corallum, the hydrauths 

 protrude and present a well-marked polymorphism. The 

 pores are arranged in groups consisting of a central one sur- 

 rounded by a varying number of smaller ones. From the 

 central pore protrudes a hydranth with a circle of short ten- 

 tacles tipped with knobs containing numerous nematocysts ; 

 this is the trophopolyp or gasterozoid (Fig. 46, g). From the 

 smaller surrounding pores more elongated hydranths protrude, 

 destitute of a mouth, and with short scattered tentacles also 



