118 



COELENTERATA. 



the central stomach or gastral pouches. They are interradially* 

 placed, and have the form of four horseshoe-shaped glands, which 

 may secondarily divide into eight adradial halves. In all cases the 

 sexual cells are dehisced into the enteron and pass out by the 

 mouth. 



In connection with the genital glands there are very generally 

 present ectodermal invaginations in the form of pits of the sub- 

 umbrella surface. These pit* are for respiratory purposes, and 

 constitute the so-called sub-genital pits (Discomedusae, Fig. 100), 

 and funnels (Lucernaridae and Periphyllidae, Fig. 98). They occur 



interradially beneath the 

 genital glands. 



Asexual reproduction by 

 budding is of very general 

 occurrence in the craspe- 

 dote medusae. The buds 

 are formed, usually in 

 groups, rarely singly, on 

 the wall of the manubrium, 

 more rarely at the base of 

 the tentacles, on the cir- 

 cular canal, or on other 

 places. In the Narco- 

 medusae the budding is 

 confined to larval life, 

 when a bud-nurse is formed 

 which attaches itself para- 

 sitically to other Medusae. 

 As hinted often in the above description the medusae fall into two 

 well-marked divisions, viz., into the Craspedota and Acraspedota 

 (Acraspeda) or Acalepliae. Since the celebrated discovery of the 

 alternation of generations in the medusae by Sars we have known 

 that the former are budded off by Hydroid-polyps, the latter by the 

 Scypho-polyp or Scyphistoma. 



The Craspedote Medusae are distinguished by the following 

 characters : a smooth umbrella edge ; a muscular velum, without 

 endoderm; a double nerve-ring; gastral filaments and taenioles are 



* In describing a Scyphomedusa it is usual to speak of eight primary radii. 

 These are the radii of the marginal bodies. These eight primary radii are further 

 distinguished into four perradii and four interradii. The oral arms lie in the four 

 perradii, and the gastral ridges and gonads in the four interradii. The adradii 

 are the eight radii between the eight primary radii. 



Ov ""^ Ob 



FIG. 106. Phialidium variabile represented from the 

 under side of the umbrella. V velum ; mouth ; 

 Ov ovary ; Ob auditory vesicle ; Rf marginal ten- 

 tacles. 



