48 HYDROIDS AND JELLY-FISH 



Most of the Scyphozoa resemble Aurelia in the general features 

 of their structure, but vary considerably in certain points. Thus 

 the umbrella, instead of being saucer-shaped, may be cup-shaped, 

 or conical ; though many pass through an alternation of genera- 

 tions such as I have briefly described in Aurelia, others do not, the 

 ciliated larvae developing directly into Jelly-fishes like the parents 

 without passing through a fixed polyp stage. Lucernaria (Halidys- 

 tus octoradiatus), a small and interesting form, differs widely from 

 the rest in attaching itself by a short stalk developed from the 

 centre of the outside of the umbrella to seaweeds and rocks, and 

 can creep about or swim at pleasure. 



The Comb-bearing Jellies (Ctenophora) have no disk or um- 

 brella, and do not resemble the true Jelly-fish or medusae, either 

 in shape or movement. Generally globular or cylindrical, rarely 

 ribbon-shaped, they form a group of active, free-swimming, gela- 

 tinous, transparent animals, at times appearing in vast numbers in 

 the surface waters of the sea. The beautiful, crystal-bright, so-called 

 "Sea-gooseberry" (Hormiphora plumosa) is a familiar Comb- 

 bearer, frequently to be seen in large numbers during the late 

 spring and early summer months, swimming close in to shore, 

 hundreds often being left stranded by the receding tide. It has 

 a wonderfully bright, transparent, globular body, provided with 

 eight rows of swimming-plates or combs ; each plate consisting of 

 a comb-like band of quite large cilia, the rapid movements of 

 which present a most interesting sight. As the Hormiphora swims 

 about in the collecting-jar, we shall see that two long, feathery 

 tentacles are protruded from each side of the body, and wave 

 about in the water, only to be as suddenly retracted within their 

 sheaths. They are destitute of thread-cells or nematocysts, which 

 are replaced by a number of adhesive cells, carrying out the same 

 function as organs for the capture of prey. At one end of the 

 body is a single opening, the mouth, which communicates with 

 a gullet and stomach ; from the latter canals pass beneath the 

 swimming-plates or combs, and two canals pass from the base 

 of the stomach to the opposite end of the body, where they open 

 as two minute excretory-pores. Centrally placed between these 

 pores is a remarkable structure which functions as the nerve centre 

 as well as an " audatory " or balancing sense organ. There is no 

 budding, colony formation, or alternation of generations among 



