266 



COELENTERATA HYDROZOA 



side of the umbrella (Fig. 132, A). The mouth leads into a 

 shallow digestive cavity, from which radial canals pass through 

 the substance of the umbrella to join a ring-canal at the margin 

 (Fig. 131). 



The sense-organs of the Medusae of the Gymnoblastea are in 

 the form of pigment-spots or very simple eyes (ocelli), situated 

 at the bases of the tentacles. The orifice of the umbrella is 



guarded by a thin shelf or mem- 

 brane, as in the Calyptoblastea, 

 called the velum. The sexual cells 

 are borne by the manubrium (Figs. 

 131 and 132, A). 



There are many modifications 

 observed in the different genera 

 as regards the number of tentacles, 

 the number and character of the 

 radial canals, the minute structure 

 of the sense-organs, and some other 

 characters, but they agree in having 

 a velum, ocellar sense-organs, and 

 manubrial sexual organs. The 

 tentacles are rudimentary in Amal- 

 thea ; in Corymorpha there is only 

 one tentacle ; in Perigonimus there 

 are two ; and in Bougainvillia they 

 are numerous; but the usual number 

 is four or six. The radial canals 

 are usually simple and four in number, but there are six in 

 Lar sabellarum, which branch twice or three times before reach- 

 ing the margin of the umbrella (Fig. 132, B). 



There can be no doubt that the Medusae of many Gymno- 

 blastea undergo several important changes in their anatomical 

 features during the period of the ripening of the sexual cells. 

 Thus in Lar sabellarum the six radial canals are simple in the 

 first stage of development (A) ; but in the second stage (B) 

 each radial canal bifurcates before reaching the margin, and in 

 the adult stage shows a double bifurcation. The life -history 

 has, however, been worked out in very few of the Antho- 

 medusae, and there can be little doubt that as our knowledge 

 grows several forms which are now known as distinct species 



Fig. 131. Medusa of Oladonema, from 

 the Bahamas, showing peculiar ten- 

 tacular processes on the tentacles, 

 the ocelli at the base of the ten- 

 tacles, the swellings on the manu- 

 brium that mark the position of 

 the gonads, and the radial and ring- 

 canals of the umbrella. (After 

 Perkins.) 



