1893.] MICROSCOPICAL JOURNAL. 97 



long and somewhat wavy and sometimes slightly branched, aris- 

 ing from a ramified basal stolon and shielded with a chitinous ex- 

 ternal shell ; the stemsare terminated above by the zooid, which is 

 never covered by any skeleton, the cuticle of the stem stopping 

 short at the base of the zooid. The zooids of the colony are all 

 alike and there is no polymorphism. Each zooid presents a broad 

 basal portion bearing numerous basal tentacles in a circle. Within 

 them is the pear-shaped body at whose base are minute stems bear- 

 ing numerous spherical medusa buds, and at whose summit is the 

 mouth, surrounded by a number of short oval tentacles. Thebodv 

 and stark are both composed of cells arranged in two layers, as in 

 the other cases already described. The medusa buds become sep- 

 arated from the colony, not, however, in the form of a swimming 

 bell, as in Podocoryne, but in a peculiar creeping form known as 

 act inula ^ and from this the egg development takes its start. 



Obelia dichotoma* (tigs. 20, 21). 



This, a graceful hydroid, forming colonies rarely more than an 

 inch in length, covering submerged objects of all sorts in the purer 

 ocean waters, it is one of the common hydroids attached to sea- 

 weed on rocky shores of outer harbors. The colony slightly mag- 

 nified (fig. 20) presents a zigzag stem, bearing alternate zooids. 

 The zooids are very small, much smaller than in Podocoryne, etc., 

 but the same structural plan can be detected in them. A main 

 stem runs up from the stoloniferous base, and this stem is a fleshy 

 tube, covered with a horny outer skeleton. This latter is made 

 up of successive joints like each other, but leaning either way alter- 

 nately. At each joint of the cuticular stem the fleshy tubular stem 

 within gives ofl' a branch which is the special stem of a zooid. 

 This stem is covered by a ringed cuticular covering, terminated 

 with an exquisitely delicate cup, "hydrotheca" or *■' calycle," into 

 which the zooid can retreat for protection. This zooid is a feeding 

 member (hydrozooid) of a polymorphic colony. It has a circlet 

 of tentacles surrounding a central manubrium. All the fleshy 

 parts of the body are cellular, ectodermal and endodermal, and they 

 differ from Hydra in no essential respect, but only in details of 

 form. Besides the numerous hydrozooids there are occasionally 

 borne, at joints of the stem, larger bodies, composed of a vase shaped 

 cuticle, gonotheca, protecting a delicate stalk within which is open 

 below to the channels of the main stalk and on the side to numerous 

 globular buds, which are medusas in process of development, 

 which latter are to escape in tlie water as free medusae, there to 



* BiBLlo. — Brooks, Inv. Zool. , p. 30. 



Riverside, Nat Hist., p. 84. 

 Lankester, E Britt., ix, p. 560. 

 Agassiz, Seaside Stud., p. 50. 



Explanation of Drawing of Obelia. 

 Fig. 20. Obelia colony as it appears to the naked eye. 



Fig. 21. Small part ot 20 highly magnified, showing the chitinous outer skeleton of the main 

 stem and of the zooids, and the two forms of zooids, the nutritive zooid and its cover, the calycle 

 orhydrotheca, and the gonozooid or medusa producing person and its cover, the gonotheca. 



