146 PHYLUM CCELENTERA. 



Sense organs may be present, in the form of "eyes," at 

 the base of the tentacles (Ocellatre), or in the form of 

 "auditory " vesicles developed as pits in the velum (Vesi- 

 culat?e). 



The reproductive organs develop either in the manu- 

 brium or on the radial canals. The products 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 persons of many hydroids, 

 have considerable powers of migration, and may originate 

 (sometimes 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. The 

 commonest kind of free-swimming larva is the planula, which 

 is oval, ciliated, and diploblastic, devoid of an opening, and 

 usually without a central cavity. In the case of those 

 medusoids which arise as liberated sexual members of 

 a fixed asexual hydroid colony, the planula settles down, 

 loses its cilia, buds out tentacles, and develops into a new 

 hydroid. 



In many Hydromedusas, as has been already noticed, the 

 sexual persons are not set free, but remain as buds attached 

 to the parent hydroid. These fixed "gonophores" show 

 many stages of degeneration ; some, notably in the floating 

 colonies of Siphonophora, differ little structurally from true 

 medusoids, while others, as in Hydr actinia, are simply small 

 closed sacs enclosing the genital products (Fig. 77). 



Third Type <?/ CCELENTERA.- -The common Jelly-fish 

 Aurelia aurita. Class SCYPHOMEDUS^:. 



This Medusa is almost cosmopolitan, and in the summer 

 months occurs abundantly around the British coasts. It 

 swims by pulsating its disc, and also drifts along at rest 

 without any pulsations. They often occur in great shoals, 

 and hundreds may be seen stranded on a small area of flat 

 sandy beach. The glassy disc usually measures about four 

 inches in diameter, but may be twice as large. The jelly- 

 fish feeds on small animals, such as copepod crustaceans, 

 which are entangled and stung to death by the long lips. 



