90 THE HYDROIDS AND HYDROMEDUS.E 



which many may occur on one hydra, is a cone-shaped structure, con- 

 taining numerous spermatozoa which are released by rupture of the 

 testis. The sperm swim to the ova and the syngamy, or fertilization, 

 occurs, after which cysts are secreted in which the zygotes develop. 

 Draw hydras with gonads. Observe hermaphroditic specimens and 

 sections of differentiating gonads. 



THE HYDROIDS AND HYDROMEDUSiE 

 Exercise 1. — Occurrence and General Structure of Colony. 



(a) Hydroids are marine ccelenterates closely resembling the hy- 

 dras in their general structure. They live attached to rocks, seaweed, 

 and submerged woodwork. They differ from the hydras in that the 

 individuals, instead of living singly, live together in colonies com- 

 parable to the colony that would be formed if a hydra budded many 

 times and all the buds remained united to the parent body. 



(b) For this study, preserved specimens of the hydroid Obelia 

 geniculata will be used. Examine, with a handlens, a portion of a 

 colony in a watch glass of water, and, also, museum specimens of 

 entire colonies of this and other hydroids. In some of the colonies, the 

 individuals are large enough to be recognized without a lens. Ex- 

 amine the obelia with the lowest power of the compound microscope. 

 The hydranths, or feeding individuals, and the blastostyles, or repro- 

 ductive individuals, are permanent members of the colony. In addi- 

 tion, the medusae, or jellyfishes, appear on the blastostyles and consti- 

 tute a third type of individual, which becomes detached. There is a 

 division of labor among the individuals of the colony. The feeding 

 individuals, which are the most numerous, are those with tentacles. 

 Immature feeding individuals with rounded ends will be seen. The 

 reproductive individuals are the blastostyles and medusae. Notice 

 how the upright stems are fastened at the base, and how rootlike, hori- 

 zontal stems extend over the surface on which the colony is attached. 

 Draw ( X 6) , showing these general features of a portion of a colony. 



Exercise 2. — Feeding Individuals, or Hydranths. 



(c) Select a fully matured hydranth that is properly expanded, 

 and identify the parts as found in the hydra: body, hypostome, ten- 

 tacles, ectoderm, endoderm, and enteron. Notice that the body of the 

 hydranth is continued downward as a slender stem, the coenosarc, 

 which is continuous with the common coenosarc of the colony. The 

 ectoderm, the endoderm, and the cavity of the hydranth are thus con- 



