RADIATA. 33 



Finally, the union between the segments is more and more reduced, until 

 they separate like a pile of hemispherical cups, as in Diagram,/^. 6, when 

 they are seen to be separate animals in an inverted position ; in fact, the 

 young or larva? of Aurelia {pi. 76, Jig. T4). These larvse (constituting the 

 supposed genus Eiyhijra) are about a line in width, and continue growing 

 and passing through such a change as to give it the structure of the adult, 

 which it acquires when about an inch in size. It is not precisely known 

 what becomes of the polypiform head of the Strobila (Diagram, /f/. 5), but 

 the base is said to produce a new set. 



It appears from these facts that the animal (Diagram, fig. 1) hatched 

 from the egg of a medusa, does not become a free medusa, but a kind of 

 polyp, SccqyMstoma strobila, which does not produce its like, but from 

 which medusas are developed. The polypoid nurse, as it has been termed, 

 is uniformly an undeveloped female, whilst of the resulting medus??, some 

 are male and some female. The nurse, like the adult medusa, has the 

 power of increase by budding. 



The annexed fig. 1 represents an 

 individual of the presumptive genus 

 Coryne, placed in the flimily Tubu- 

 laridffi (p. 27). The head is a six- 

 armed hydroid, beneath which are 

 four quadrate, bell-shaped bodies, 

 which are not organs, but distinct 

 1 2^3 individuals of an entirely different 



form from the hydroid. In the concavity of each is suspended a quadrate 

 stomach, as shown in fig. 2. These bodies have an independent motion, 

 sucking the water in, and throwing it out like the Medusae. They finally 

 detach themselves, and swim freely like medusse, to which they bear a close 

 resemblance. Steenstrup, who observed this species in Iceland, found 

 larger individuals {fig. 3), which he considers the adult medusaform of the 

 former, in which one of the angles bears a lobed organ and two threads, 

 which he regards as female generative organs. Steenstrup regards Coryne 

 as " a previous generation of preparative nurses, which are so far asexual, 

 inasmuch as that their generative organs are not developed." 



Forbes describes two minute British species allied to fig. 3, under the 

 generic name of Steenstrupia, suggesting that they may be a stage in the 

 history of some hydroid form. 



Class 2. Zoo2)hyta. 



The Zoophyta are chiefly marine ; some species are sedentary and others 

 free, some live as single independent animals, and others are collected 

 together in large colonies, the base of the stems being united. Some are 

 without a hard sujjport, others secrete a stony skeleton, which is named 

 coral (coKALLUM, coKALLA in the plural). 



The corallum is not usually external like the shell in the MoUusca, as is 

 popularly supposed, but an internal secretion " entirely concealed," in the 



237 



