JELL Y-FISHES. 



97 



has no known differences from the parents which produced the egg or spermatozoon. 

 The principle is a wide-spread one in the animal kingdom, and is known as the 

 alternation of generation. It is evident that the 



Scyphistoma and Strobila, more especially the [>, [\ 



latter, have a wide difference in shape from f^^^ ^ '-'•' ' 



the form of the adult Cyanea. They develop 

 directly from the egg and are asexual, while 

 the adults which are developed from them are 

 sexual. Sexual animals produce ova which de- (^ 

 velop into Strobilte as before. Here then is an 

 alternation of sexual with asexual forms of the 

 same animal, and the technical name of the 

 anomalous development is " Alternation of Gen- 

 erations," nowhere better illustrated than in the 

 Hydroidea and Discophora. 



The devek)i)ment of the ovum of Ci/anea into i. , „, -c^ ^^^ , ^ ,„ ■ , , 



' •' Fig. 91.— Ephyraof ^Krc/to.Sai-irfM/a. 



the adult by a process of alternate generation, in 



which intennediate larva? are fixed to some foreign body and reproduce the adult by 

 self-division, is not found in all the Discophora. As this method of growth may- 

 be said to be indirect in character, another, called the direct fi-om the absence of 

 tliese intermediate asexual conditions, also exists. In a direct development among 

 the discophorous medusae we have sunply a continuous gi-owth from the egg to the 

 adult. One egg produces only one adult. Such a development takes place in Pelagia 

 and one or two related genera. 



Class in. — SIPIIOXOPHORA. 



Among the most beautiful of all the medusae is the group called the Siphonophora, 

 the tube-like jelly-fishes. These animals are all marine and free swunming, and 

 although they often have a hydroid-like shape, which resemblance becomes more marked 

 when we study their anatomy, they are never attached to the ground as are the mem- 

 bers of the Hydroidea. They are found in all oceans, although the tropics seem to be 

 richest in the variety of these animals, and those from the Mediterranean have up to 

 the present time been the most carefully studied and described. 



As their name signifies, the Siphonophora are characterized by a tulje-like body, 

 which is generally so much elongated tliat it takes the form of a small axis or stem. 

 Although there are several genera in the gi'oup where the body does not assume a 

 tubular fonn (of which one of the most common is Pht/salia), a tubular body seems 

 as a rule characteristic of the grouji. 



The relationships of the Siphonophora to other medusie have been variously inter- 

 jireted by different authors. By the majority they are regarded as comjiarable to the 

 Hydroidea, and are often called the free-swimming hydroids, in distinction from those 

 already considered which are fixed. Others still compare them with the gonophores 

 of the hydroids, some of which as the genus Lizzia bud off from the side of their 

 manubrium new individuals, which later develop into medusce like their parent. The 

 Siphonophora would be regarded T)y them as similar to the parent with many attached 

 young. While many facts can be mentioned in sn])port of either of these theories, it 

 may be said that the differences which exist between a free medusa and an attached 



