Some of the Lozver Metasoans 393 



Gonionemus. — The medusa of this form is a small jellyfish about 

 one-half inch in diameter. It possesses the typical bell-shaped structure 

 and has a number of hollow tentacles along the margin. The convex 

 aboral surface is called the exumhrella, while the concave oral surface 

 is the suhiimhrella. The subumbrella is partially closed by the projecting 

 shelf of tissue, the vehini, which is around the margin of the umbrella. 

 Also around the margin are many sense organs derived from sensory 

 epithelial cells. These are the statocysts w'hich are concerned with the 

 balance of the organism. Also on the subumbrella surface is the manu- 

 brium, a central projection of tissue which has the mouth and its sur- 

 rounding lobes at its end. The mouth opens into the central gastro- 

 vascular cavity. This cavity is connected by four radial canals with the 

 ring canal that circles the edge of the umbrella. 



From the tissue below the radial canals are suspended the gonads. 

 These forms are dioecious ; thus a single animal produces only eggs 

 or sperm. The fertilized tgg develops into the typical planula larva 

 which swims about for a bit before settling down to become a small, 

 inconspicuous polyp. This soon buds ofif the meduosid forms. 



The Siphonophores. — Colonies of hydrozoans may become ex- 

 ceedingly complex in their growth patterns. The extreme of this 

 polymorphism is exhibited by the members of the order Siphonophora. 

 The Portuguese man-of-war, Physalia (Fig. 131,a), is one of these. A 

 large medusoid form serves as a gas-filled float and from it are suspended 

 numerous polypoid individuals. Some of these latter are defensive, 

 others are nutritive, and still others are reproductive. Some colonies be- 

 come very large and may have a float as much as ten inches in length, 

 with stinging polyps up to sixty feet in length. Often these forms are 

 beautifully colored and quite spectacular to see although they can be 

 extremely dangerous to any swimmer who chances to come in contact 

 with them. Their viscious stinging polyps release numerous poisonous 

 nematocysts which cause festering wounds. A relationship has resulted 

 between certain species of small fish and the siphonophores. These 

 small fish live among the stinging tentacles and dash out for their food. 

 Apparently they are not harmed by the stinging polyps and actually are 

 well protected from their larger enemies. 



THE CLASS SCYPHOZOA 



The class Scyphozoa is characterized by the fact that its members 

 have as the conspicuous stage in the life history the medusoid form. 



