ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



type of hydroid without a hydrotheca is said to be gymnoblastic, in con- 

 tradistinction to the calyptoblastic type in which a hydrotheca is present. 

 Tubularia belongs to a group in which the Medusae (Fig. 41, B) 

 show important differences from those of Obelia. (i) They are deep 

 bell-shaped instead of saucer-shaped ; (2) the gonad {g) is situated not 



A 



r.c. 



Fig. 41. 



Diagram to illustrate tiie distinguishing features of A, Gymnoblastic polyp ; B, Antbomedusa ; 

 C, Calyptoblastic polyp; D, Leptomedusa. g, Gonad; h, hydrotheca; m, mouth; ps^ perisarc; 

 r.c, radial canal ; s, stomach ; v, velum. [In Figs. B and D the right half cf the figure is in the plane 

 of a radial canal : the left half is in a plane between two radial canals,] 



on the under surface of the umbrella but on the outer surface of the 

 manubrium ; (3) the sense-organs when present are usually in the form 

 not of otocysts but of simple eyes. Such a type of medusa is known as 

 an Anthomedusa (Fig. 41, B) in contradistinction to the Leptomedusa 

 (Fig. 41, D) as exemplified by Obelia. In the genus Tubularia the 

 medusoid generation is apparently commencing to degenerate, as the 



