COELENTERATA 6i 



style^ which Is a continuation of the living central portion, the 

 coenosarc. The medusa buds, when mature, pass out at the top 

 of the gonangium and develop into medusae. Some medusae pro- 

 duce eggs and some produce sperms. The fertilized eggs develop 

 into a motile stage which, after swimming around for a time, settles 

 down and grows into a colony similar to the parent. The colonial 

 form reproduces asexually, by budding, and the medusae reproduce 

 sexually, by eggs and sperms. The zoophyte stage begins in the 

 autumn, and the medusa stage in the spring, so the life history takes 

 one year. 



Siphonophora. — While we cannot discuss all the Orders of 

 Hydrozoa, we will consider briefly one of the most beautiful of the 

 pelagic forms, belonging to the Order Siphonophora. The familiar 

 " Portuguese Man of War " (Figure 24) consists of a colony of 

 individuals illustrating the condition of polymorphism. The 

 ectoderm invaginates and produces a large pneumatophore or float. 

 From the coenosarc arise individuals functioning as sensory polyps 

 or feelers, and numerous retractile tentacles supplied with nettle 

 cells. Feeding tubes digest and distribute the food. Reproductive 

 zooids are also present. 



Class 2. Scyphozoa. — All Scyphozoa are marine, the majority 

 being pelagic, i.e., swimming at the surface of the ocean. Some of 

 them are beautifully colored, and certain species are phosphorescent. 

 All jellyfishes are carnivorous, and the larger forms are able to 

 capture and consume fishes. In their life history alternation of 

 generations is found, but the asexual stage is not highly developed 

 and in some cases a simple metamorphosis occurs. 



Type of the Group — Aurelia. (Figure 25.) External Char- 

 acteristics. — Aurelia is a saucer-shaped jelly fish about 4 inches in 

 diameter, with four distinctive gastric pouches, conspicuous because 

 of the orange gonads found inside them arranged along the outer 

 wall towards the margin. The concavo-convex umbrella has an 

 exumbrella only slightly elevated in comparison with Gonionemus, 

 and the velum is absent. Each gastric pouch has a subgenital pit. 

 These have no connection with the extrusion of embryos and prob- 

 ably are respiratory and excretory. 



Digestive System. — The mouth, rectangular when distended. Is 

 usually collapsed In the preserved specimen. Four oral arms, folded 

 like a leaf, and although they are devoid of tentacles, plentifully 

 supplied with nematocysts, are used to transport food to the mouth, 



