90 CCELENTEBATA 



and in 1812 brought polyps and medusae together in the single type- 

 group of Animalia-Radiata. 



The next few years saw an immense increase in the knowledge of 

 both the anatomy of the medusa and the polyp and in the number of 

 the forms knoAVTi, yet it was not until 1841 that M. Sars, on the basis 

 of his studies of Aurelia aurita, and 1842 that Steenstrup, on that of 

 his studies of Coryne, could first definitely formulate the principle of 

 the alternation of generations in cnidarians and elucidate the relation of 

 the polyp to the medusa. Even as late as 1837 Loven held the polyp and 

 medusa to be dimorphic sexual forms, the former being the male and 

 the latter the female individual. In 1847 Leuckart created the phylum 

 Ccelenterata and called attention to the fundamental sack form of the 

 body, and in 1849 Huxley showed that the walls of this sack were made 

 up of two layers which he named ectoderm and entoderm and homologized 

 with the two primitive germ layers of the higher animals. In 1851 Vogt 

 introduced the useful term Hydromedusa, Huxley in 1856 that of Hydro- 

 zoa, and Claus in 1891 that of Scyphozoa. The latest development of 

 the system is due to many authors, of whom perhaps Chun and A. G. 

 Mayer are especially to be mentioned. The first important American 

 work on cnidarians was J. D. Dana^s Export of the Zoophyta of the 

 Wilkes Expedition (1846). Louis and Alexander Agassiz and their 

 pupils and followers have done the most to extend the knowledge of 

 American cnidarians. Mayer^s monograph, TJw Medusae of the World, 

 is the most important recent work. The subphylum contains about 4,200 

 species, grouped in 3 classes. 



Key to the classes of Cnidaria: 

 Oi Small hydroid polyps and both small and large medusae. 



b, Hydroids without mesentaiial ridges and usually colonial, and craspedote 

 njg(3usae 1- Hydrozoa {Hydromedusae) 



6, Minute hvdroids with 4 mesentarial ridges, and acraspedote medusae. 



2. ScYPHOZOA (Scyphomedusae) 

 a. Corals, sea-auemones and gorgonians 3. Anthozoa 



Class 1. HYDROZOA. (Hydromedusae.) 

 Hydroid polyps and craspedote medusae, usually with alternation of 

 generations. The hydroid stage, which is called the trophosome, is sessile 

 and usually colonial and produces by budding the medusoid stage, which 

 is called the gonosome. The latter is sexual and either male or female. 

 The individual hydroids are small, being usually but a few millimeters 

 in length, the solitaiy tubularians being exceptions, which may be several 

 centimeters, and in the case of the deep-sea Branchiocerianthus imperator, 

 which is allied to Corymorplia, a meter or more in length. The colonies 

 are often plant-like in appearance; the individual polyps are called the 



