24 MORPHOLOGY OF INVERTEBRATE TYPKS 



straight, divided into regular inkrnodes each with a pair of 

 opposite- hydranths. Branches are always opposite, arising at 

 the base of the hydranths. Every intcrnode giving rise to 

 branches has therefore two hydranths and two branches. The 

 ramuli or branches of the second order arise asymmetrically, 

 /. e., one ramulus to an internode of a branch. The colony is 

 largely composed of trophosomes, i. e., common hydranths whose 

 main function is nutrition, and a few gonosomcs or modified 

 polyps which depend for their nutrition on the trophosomes and 

 whose function is sexual reproduction. The gonosomes are 

 asymmetrical, a single gonosome being produced by an internode 

 in front of the trophosomes. 



Trophosome. We have already seen that the hydranths 

 of Sertularia pumila are protected by a theca. As there are two 

 hydranths for each internode, there are also two hydrothcccc 

 for each internode. They sit on a "shelf" of the internode and 

 the base of each hydrotheca presents a regular diaphragm with 

 a circular opening the diameter of which is considerably smaller 

 than the diameter of the hydrotheca itself. The ccenosarc of 

 the stem is therefore distinctly constricted by the diaphragm, 

 through which it has to pass to form the polyp. The free end 

 of the hydrotheca is provided with an oval opening or aperture. 

 The edge of the aperture is emarginate, produced into two 

 opposite teeth. The aperture may be closed by a two-flapped 

 opcrculum or lid when the hydranth is entirely withdrawn into 

 the hydrotheca. This operculum is best likened to a double door 

 only that the so-called adcaulinc flap, i. c., the one whose hinge 

 is nearest to the stem, is much smaller than the abcaitUnc flap. 

 The hydranth is provided with a special cctodcrnial fold which 

 arises considerably below the tentacles and is attached to the 

 edge of the hydrotheca. This fold forms an almost complete 

 contractile sheath and serves as a protractor. 1 The hydranth 

 has a single circle of usually sixteen filiform tentacles situated at 

 the base of the hypostomc. They have the shape of the basal 

 1 Nulling mistook the optical section of the sheath for two protractors. 



