STRUCTURE OF A MEDUSOID. 131 



The contractions of ectodermic muscle-cells on the under (sub- 

 umbrellar) surface of the bell cause slight alterations in shape, and thus 

 the medusoid moves. Similar cells effect the movement of manubrium 

 and tentacles. 



The nervous system consists of a double ring of nerve-fibres around the 

 margin of the bell. With these are associated ganglionic cells, which 

 apparently control the muscular contractions. 



Medusoids derived from Tubularian hydroids have eyes at the base of 

 the tentacles, and are therefore called Ocellatse. Those derived from 

 Campanularian hydroids have auditory vesicles developed as pits on the 

 velum, close to the inner nerve-ring, and are therefore called Vesiculatte. 



The reproductive cells develop by the side of the manubrium in 

 ocellate medusoids, at the base of the manubrium or on the course of a 

 radial canal in vesiculate medusoids. They always ripen in the 

 ectoderm and often seem to arise there ; but Weismann and others have 

 shown that the reproductive cells of a medusoid derived from a hydroid, or 

 of the reduced and fixed reproductive person of many hydroids, have con- 

 siderable powers of migration, and may originate (sometimes apparently 

 in the endoderm) in the hydroid colony, at some distance from the place 

 where they are matured within the medusoid bud. The sexes are usually 

 separate, and the commonest kind of free-swimming larva is a planula- — 

 a closed oval sac with two layers of cells, of which the outer are ciliated. 

 In those medusoids which arise as the liberated sexual persons of a fixed 

 asexual hydroid colony, the planula settles down and develops into a new 

 hydroid. 



The class Hydrozoa thus includes (a) Hydra and its 

 relatives (Hydridae) ; {V) hydroids and medusoids (Hydrome- 

 dusae), whether both these forms be united in one life-history 

 or not ; but it also includes (c) a third order (Siphonophorae) 

 of free-swimming medusoid colonies, with much division of 

 labour, of which the Portuguese Man-of-War {Physalia) and 

 Velella are instructive types. Some medusoids which multiply 

 by budding, suggest how these Siphonophorae might arise. 

 The order is of great interest because of the division of 

 labour which is illustrated among the members of colony. 

 Thus there are usually nutritive, reproductive, sensitive, and 

 natatory " persons " in one colony. 



Sub-class, Scyphomedusae or 

 B. Class ScYPHOzoA — -^ Acraspeda. 



Sub-class, Anthozoa or Actinozoa. 



The common Jellyfish — Aurelia aurita, one of the 

 Scyphomedusae. 



This medusa is almost cosmopolitan, and in the summer 



