1893.] MICROSCOPICAL JOURNAL. 97 



lon^ and somewhat wavv and sometimes slji^htlv l)i;mclie(l, aris- 

 ing; tVom a ramified basal stolon and shielded with a chitinous ex- 

 ternal shell ; the stemsaic terminated above bv the zooid, whicii is 

 never covered by anv skeleton, the cuticle of the stem stoppinj^ 

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

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

 basal portion beariiiLj numerous liasal tentacles in a circle. Within 

 tiiem is the pear-shaped bodv at wiiose base aie minute stems bear- 

 inj^ numerous spherical medusa l)uds, and at whose sumiT)it is the 

 mouth, surrounded by a number of short oval tentacles. The bodv 

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

 the otiier cases alreatlv descrilied. The medusa buils ])ecome sep- 

 arated tVom the colony, not, iiowever, in the form of a swimming 

 bell, as in Poilocoryne, but in a peculiar creeping forin known as 

 acthiiila, and fr(jm this the egg development takes its start. 



ObELIA DICHOTOMA* (figS. 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 rockv shores of outer harbors. The colony slightlv mag- 

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

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

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

 stem runs uj^ 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 andendodermal. and they 

 dirt'er 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 ofa 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 medusae in process of development, 

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



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



Riverside, N.-\t Hist., p. 84. 

 Lankesler, E Britt., ix, p. 560. 

 Agassiz, Seaside Stud., p. 50. 



Explanation of Drawing of Obblia. 

 Fig. 2Q. Obelia colony as it appears to the naked eye. 



Fig. 21. Small part ol 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 

 or hydrotheca, and the gonozooid or medusa producing person and its cover, the gonotheci. 



