Jelly-fish 



39 



in relation to the automatic or instinctive 

 behaviour of these animals. 



The Medusoids are generally bell-shaped. 

 As a typical example of these animals, we may 

 refer to a jelly-fish known as Sarsia (Fig. 3), 

 which may be said to 

 consist of a tubular 

 body and manubrium 

 with a mouth or open- 

 ing, from which a pas- 

 sage leads to its diges- 

 tive cavity. From this 

 cavity canals pass out- 

 wards to terminate in a 

 passage which extends 

 round the margin of 

 the bell. In Sarsia 

 and many other Medu- 

 soids a shelf or velum 

 projects inwards from 



the margin of the bell, and at the junction 

 of the velum and rim of the bell tentacles hang 

 downwards. (Fig. 3.) 



The ectoderm or outer surface of this Medusa 

 is formed by a layer of flattened cells. Mus- 



FIG. 3. Diagram of a Medu- 

 soid (Sarsia). m, manubrium ; 

 es, external surface of bell ; 

 sb, subumbral surface ; v v, 

 velum ; su, subumbral cavity ; 

 cl, circular canal ; t t t, ten- 

 tacles. (After Lankester's 

 Treatise on Zoology, Part II., 

 p. 17, The Hydromedusae.) 



