CHAPTER SIXTH. 



THE CORALLARIA TABULATA AS IIYDROIDS, 



SECTION I . 



MILLEPORA ALCICORNIS LINN. 



The hydra. — Until now, the genus Millepora has been referred to the class of 

 Polyps, and so long as the soft parts of the animals forming tliis kind of Corals 

 remained unknown, there ajipeared no reason why they should not be associated 

 with the Coral-builder, even after it had been ascertained that the Bryozoa belong 

 to the branch of MoUusks. The opportunity I had while in Florida of observing 

 Millepora alive, has satisfied me, however, that this genus has none of the char- 

 acteristic features of the true Polyps, the main cavity of the body not being 

 divided by vertical radiating partitions into chambers, as is the case in all the 

 members of this class. Like the true Hydroids each individual has a simple, 

 undivided cavity, with double walls. The individual hydra? (PI. XV. Figs. 4, 5, 

 and 6) resemble very strikingly those of Halocharis (PI. XX. Fir/. 10), and, to some 

 extent also, those of Coryne (PI. XVIL), and even those of the fertile Hydractinia 

 (PI. XVI.). As in these genera, the mouth (PI. XV. Figs. 5 and 6, d) opens at the 

 summit of the head, as a simple, round aperture, alternately opening and closing; 

 the digestive cavity being a simple straight cylinder when empty {Fig. 6, c), and 

 widening somewhat when full {Fig. 5, c^). The outer wall (a) is much thinner 

 than the inner wall {h), which consists of large cells {Fig. 7, e), stretching across 

 the whole thickness of the wall There are two kinds of Indra^ {Fig. 4) in one 

 and the same community; the large ones {Figs. ^, d g h, and G), with very few, 

 and generally only four or five, and seldom six, short tentacles, around the head, 

 terminating in a more or less spheroidal knob {Fig. 6, e / g), supjjorted by a short 

 peduncle {h), are fewer in number ; the smaller ones are much more numerous. 



