JELLY-FISHES. 



97 



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

 The princijDle 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 

 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 Strobilae 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 development of the ovum of Cyanea into 

 the adult by a process of alternate generation, in 

 which intermediate larvae 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 from the absence of 

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

 the discophorous medusae we have simply a continuous growth 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. 



Fic. 91. — Ephyra of Aurelia fiavidula. 



Class III. — SIPHONOPHOKA. 



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

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

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

 Although there are several genera in the group where the body does not assume a 

 tubular form (of which one of the most common is Physalia), a tubular body seems 

 as a rule characteiistic of the group. 



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

 preted by different authors. By the majority they are regarded as comparable to the 

 Hydroidea, and are often called the frecnswimming 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 medusae like their parent. The 

 Siphonophora would be regarded by them as similar to the parent with many attached 

 young. While many facts can be mentioned in support of either of these theories, it 

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



VOL. I. — 7 



