86 



LOWER INVERTEBRATES. 



The gonothecae are developed on the branches and are elongated, somewhat urn- 

 shaped, aperture central, terminal with usually two, occasionally one long horn at the 



anterior end. The eggs are partly devel- 

 oped within the gonotheca, and then pass 

 into a sac which projects from the orifice 

 of the gonophore, where they finally be- 

 come planulse ; these after living a free 

 life become attached and form a new col- 

 ony. Sertularia argentea is found from 

 the New Jersey coast to the Arctic Ocean 

 from low-water mark to a depth of over 

 one hundred fathoms. It is very widely 

 distributed, being found on both sides 

 of the Atlantic and on the Pacific shore 

 of the United States. Like many other 

 Hydroids it is often col- 

 lected as sea-moss and is 

 not infrequently seen at 

 the florists for decorative 

 purposes. 



Another very common 

 species of iSertularia is 

 IS. p^imila, a very much 

 smaller hydroid, not over 

 one inch and a half long, 

 and often found in abun- 

 dance on the common 

 Fiicus or dark brown 

 rock-weed. The hydro- 

 thecse are opposite one 

 another on the stem, giv- 

 ing it a compactness of 

 structure and regularity of outline not 

 possessed by S. argentea. The colonies 

 ' are sexually perfect from May to Sep- 

 tember on the New England coast. Tlie 

 method of reproduction is very similar to that of S. argentea. 



A third large family comprises the feathery forms known as the PLtrMtrLAEiD,E. 

 They are represented on the New England coast by Plumularia tenella, Plumularia 

 verrillii, and Aglaophenia arhorea ; the last species was described by Desor in 1848, 

 and has, I believe, never been found since. Other species of Aglaophenia and Phtm- 

 nlaria are found on the Carolina coast, and. still others in the Californian waters. 

 Perhaps the most elegant in appearance of all the American hydroids is the ostrich 

 plume of our Pacific coast, Aglaophenia struthionides. It varies much in size and 

 color, but always retains the appearance of a diminutive ostrich feather. Microscopic 

 study shows that the hydrothecaa ai-e arranged in a single row on one side of each 

 branch or pinna, and that the branch is divided into very short joints, one to each 

 hydrotheca. Each hydrotheca has its rim ornamented with a number of sharply 



Fig. 79. — A frag- 

 ment of Sertw- 

 laria, enlarged, 

 showing the cups 

 or hydrotheca. 



Fig. 78. — Sertularia argentea, natural size. 



