168 ZOOLOGY sect, iv 



This is still more notably the case in the lower Metazoa, such 

 as Ascetta and Hydra, in which we have numerous cells combined 

 to form a permanent two-layered sac with a terminal aperture, 

 some of the cells having digestive, others tactile, others repro- 

 ductive functions. Thus while an Amoeba or a Paramcecium is 

 an individual of the first order, Hydra and Ascetta are individuals 

 of the second order, each the equivalent of an indefinite number of 

 individuals of the first order. 



In the Hydrozoa we see this process carried a step further. 

 Budding takes place and colonies are produced, the various zooids 

 of which — each the equivalent of a Hydra — instead of remaining 

 all alike, become differentiated both morphologically and physio- 

 logically, so as to differ immensely from one another both in form 

 and function. In Obelia, for instance, reproduction is made over 

 exclusively to the medusae, while in Halistemma we have zooids 

 specially set apart, not only for reproductive, but for tactile and 

 protective purposes. Thus in Halistemma and the other Siphono- 

 phora there is a very complete subordination of the individual 

 zooids to the purposes of the colony as a whole, the colony thus 

 assuming, from the physiological point of view, the characteristics 

 of a single individual, and its zooids the character of organs. In 

 this way we get an individual of the third order, consisting of an 

 aggregate of polymorphic zooids, just as the zooid or individual 

 of the second order is an aggregate of polymorphic cells or 

 individuals of the first order. 



CLASS II.— SCYPHOZOA. 



1. Example of the Class — The Common Jelly-fish 



(Aurelia aurita). 



( Aurelia is the commonest of the larger jelly-fishes, and is often 

 found cast up on the sea-shore, when it is readily recognisable by 

 its gelatinous, saucer-shaped Umbrella, three or four inches in 

 diameter, and by having near the centre four red or purple horse- 

 shoe-shaped bodies — the gonads— lying embedded in the jelly. 



External Characteristics. — The general arrangement of the 

 parts of the body is very similar to what we are already familiar 

 with in the hydrozoan jelly-fishes. Most conspicuous is the 

 concavo-convex umbrella, the convex surface of which, or ex- 

 umbrella, is uppermost in the ordinary swimming position (Figs. 

 126 and 127, A). The outline is approximately circular, but is 

 broken by eight notches, in each of which lies a pair of delicate 

 processes, the marginal lappets (mg. Ip.) : between the pairs of-, 

 lappets the edge of the umbrella is fringed by numerous close-set ) 

 marginal tentacles (t). 



