2fi 



MEDTS.E (>K THE WOULD. 



Hydroid. The hydroid is very abundant upon eel-grass, seaweeds, stones, or wharves, 

 below low tide. The stems arise from a ramifying stolon. They attain a height of 100 to 125 

 mm. The main stems branch alternately, the longest and oldest side branches being found 

 nearest the base of the stock. The side branches give rise to simple or slightly branched 



ramuli from their upper sides. These ultimate 

 ramuli are annulated at base with 5 to 6 rings, and 

 the main stem and primary branches are annulated 

 beyond each branch. The stems are covered with 

 a chitinous, horny perisarc. The polyp-heads are 

 flask-shaped, and the mouth is situated at the ex- 

 tremity of a long, slender, conical throat-tube. 

 There are 2 or 3 verticils of oral tentacles, each 

 row being composed of 5 to 7 short, stiff tentacles, 

 each terminating in a knob-shaped cluster of nem- 

 atocysts. Besides these there is a single circlet of 

 12 to 16 long, flexible tentacles near the proximal 

 base of the polypite. The medusa-buds develop 

 upon the sides of the polypites between the oral 

 and basal rows of tentacles. There are usually not 

 more than 2 or 3 buds upon each polypite at the 

 same time. The entoderm of this hydroid is white 

 to rose-colored, the ectoderm silvery and translu- 

 cent, and the perisarc horny-yellow to black. 



This hydroid ranges from the West Indies and 

 Bermudas to the coast of Maine. Pentiaria gibbosa 

 L. Agassiz, of the Florida coast and West Indies, 

 is probably identical with P. tiarella, but the 

 medusae of the southern form wither on the stem 

 as do those of P. Jisticha. Indeed Agassiz's figure 

 in 1862, shows the ultimate pedicels of his P. gibbosa, from Key West, Florida, ringed 

 throughout as in the Mediterranean Pennaria disticha. 



Hargitt, 1900, has made a study of the life-history and development of P. tiarella. Early 

 in the summer the hydroids are found growing on rock-weed, piles of docks, etc.; while late 

 in the summer they take to eel-grass. The early summer brood is not so bright in color, and 

 the medusae mature more slowly than in the late brood. The individual hydranths of the 

 late brood are, however, smaller than those of the early summer brood. The medusae dis- 

 charge their ova soon after liberation, and then die rapidly. The ova are 0.4 to 0.5 mm. in 

 diameter and are heavily laden with yolk. They are creamy-white to orange in color. The 

 cleavage is found by Hargitt to be subject to much individual variability, so that an extra- 

 ordinarily irregular, loosely connected mass of cells is formed, resembling the condition 

 described by Rittenhouse, 1907, in Turritopsis nutricula. No matter how irregularly shaped 

 the embryo may be, it results finally in the formation of a spherical solid morula, and this 

 soon changes into a pear-shaped, ciliated, planula larva. In 5 to 7 days after the beginning 

 of development, the larvae settle down and then develop into small monogastric hydroids 

 covered with ringed perisarc, and provided with whorls of tentacles as in the adult. Hargitt 

 finds that the removal of small parts of the segmenting eggs does not alter the future history 

 of development. The eggs may also be divided at the first or second cleavage and will still 

 develop into normal larvae. Moreover, Hargitt finds that single eggs sometimes separate 

 normally into two masses, each of which gives rise to a normal embryo. The irregularities 

 in cleavage observed by Hargitt have been seen also by Miss Cora J. Beckwith. She finds 

 that the segmentation is mytotic, not amitotic; the nuclear division constantly outnumbering 

 the cytoplasmic so that a syncytium is formed. 



Hargitt, 1901, states that hydroid stocks of Pennaria which grow upon eel-grass in shallow 

 water are more pinnatified in their branching, and more highly colored than are those which 

 grow upon stones, etc., in deep water. He also finds that the medusae of the deep-water 

 hydroids are less active than are those developed upon the hydroids which grow in shallow 



Flo. 2. Southern variety of Pennaria tiarella. From 

 life, by the author, Tortugas, Florida, May 

 27, 1908. 



Branch of hydroid and enlarged view of a polypite 

 showing pedicels ringed only at their bases. 



