Chap. II. LIMITS OF THE CLASS. , 99 



in any way, while it is true that the individuals of one and the same species 

 are more or less polj'morphous. The circumstance that naturalists may find it 

 difficult to trace the natural limits of any one 2:)articular species, or the mistakes 

 they may make in their attempts to distinguish them, has nothing whatsoever to 

 do with the question of their origin. 



There is another feature of the species of Acalephs, which deserves particularly 

 to be noticed. All these animals are periodical in their apjiearance, and last for 

 a short period in their perfect state of development. In our latitvide, most Me- 

 dusfB make their appearance as Ephyrte early in the sjjring, and rapidly enlarge 

 to their full size. In September and October, they lay their eggs and disappear; 

 while the young hatched from the eggs move about as Planulte for a short time, and 

 then become attached as Scyphostomes, and j^ass the winter in undergoing their 

 Strobila metamorphosis. The Ctenoj^horaj appear also very early, and lay their 

 eggs in the autumn, passing the winter as young, and growing to their full size 

 towards the beginning of the summer. Among the Hydroids there is more 

 diversity in their periodicity. Hydraria are found all the year round ; but the 

 Medusa? buds, the free Meduste, and the Medusaria make their appearance in 

 different seasons, in different species. Some bring forth Medusa? buds and free 

 Medusa? or Medusaria during winter ; others, and in our latitude this is the case 

 with by far the largest number of the Hydroids, produce their Medusa? brood in 

 the spring ; while a few breed later, in the summer or in the autumn : so that, 

 notwithstanding the regularity of their periodical return, Acalephs may be studied, 

 in some condition or other, during the whole year. 



SECTION VI. 



NATURAL LIMITS OF THE CLASS OF ACALEPHS. 



The principles upon which the natural limits of this class may be determined 

 have already been discussed (Sect. I. p. 36). They are based upon our knowledge 

 of the structure and embryonic growth of these animals. Upon both points our 

 information is both satisfiictory and sufficient. The anatomical researches of the 

 writers quoted from p. 18 to p. 28, and the additional facts I have traced in 

 the preparation of this monograph, cover the ground sufficiently to open a fair 

 view, not only into the general structure of the whole class, but also into the 

 correspondence of the structural features of the different groups of the class, as 

 compared with one another. The same may be said of the embryology of these 



