HYDROIDS. 



83 



In the form known as Z,izsia, it is the jelly-fish itself that produces the medusa 

 buds. In our figure, which represents the young of X. octopunctata, may be seen 

 younger jelly-fishes budding from 

 the sides of the proboscis of the 

 parent, and frequently in life, one 

 can see still younger buds in these 

 embryos before they free themselves 

 from the parent. When arrived at 

 a moderate size, these buds begin 

 their contractions and struggles 

 which finally end in their breaking 

 loose from the parent, and the be- 

 ginning of life on their own account. 

 With age and increasing size, the 

 tentacles grow much longer, those 

 arising opposite the radial canals 

 being in bunches of five, while 

 those at the intermediate points 

 are in threes, so that there are 

 thirty-two in all. 



Here also belongs the genus Stomatoca, with its two long, marginal tentacles. In- 

 confinement our S. apicata seems to prefer the bottom of the aquarium, and but rarely 

 comes to the surface. 



Fig. 75. —lAzzia octopunctata, young. 



Sub-Oeder III. — Caxyptoblastea. 



Nearly all the many species of Hydroids on the American coast which have bell- 

 shaped hydrothecsB, belong to the large family Campanulaeid^. One of the finest 

 representatives in American waters of this family of hydroids is Obelia longissima. 

 It lives in shallow water, and down to a depth of about twenty fathoms, from Long 

 Island Sound to the Bay of Fundy. The colonies are often quite large, measuring 

 eight to twelve inches in length, and are of great beauty ; at the tip of each stem 

 and branch is developed a zooid, and about the zooid is a cup of chitin, called 

 hydrotheca, into which the zooid may nearly or completely retract itself, and out 

 of which it may stretch and unfurl its single wreath of tentacles ; the rim of the 

 hydrotheca is cut into a number — twelve to sixteen blunt teeth ; the proboscis is 

 very large and very mobile, constantly changing shape. In the axils of the branches 

 are developed other chitinous cups (gonothecse) larger and always of a different shape 

 from the hydrothecse, in each of which there is a long, simple zooid, destitute of mouth 

 and tentacles (a blastostyle) ; on its sides are produced small buds, from eighteen to 

 twenty-four, which develop into medusae. They are found escaping from the gono- 

 thecse from April to June. 



These medusa are sexual, and bear either male or female elements along the radial 

 canals ; each fertilized egg develops into a ciliated planula, and this gives rise to a col- 

 ony of Obelia longissima. One finds certain points of difference between this medusa 

 and that of Pennaria. The contracting wall which subserves the function of locomo- 

 tion is not bell-shaped, but is nearly a flat disk, and tentacles exist all round the edge 

 of the disk, there being from twenty to thii-ty, while the medusa of Pennaria has only 



