HYDROZOA 143 



Order TRACHYLINA 

 This group consists of forms in which the medusoid develops directly 

 from the egg and the polyp has disappeared from the life cycle. The 

 possession of sense tentacles is an important character. It consists of 

 two suborders: 



Trachomedusae. Trachylina with sense tentacles in pits or vesicles 

 and with gonads situated in the radial canals ; with marginal tentacles 

 on the edge of the umbrella. Examples: Geryonia^ Limnocodium , 

 Carmarina (Fig. 117 II), Limnocnida. 



Narcomedusae. Trachylina with sense tentacles not enclosed and 

 marginal tentacles inserted some distance aborally from the edge of 

 the umbrella ; with gonads on the oral wall of the stomach. Example : 

 Ciinina. 



The inclusion of the following freshwater forms in the order is 

 provisional : 



Limnocnida is a remarkable freshwater form found in the Central 

 African lakes. Up till the present only male medusae have been found 

 in Lake Tanganyika and female in Victoria Nyanza. Asexual repro- 

 duction by budding takes place from the margin of the bell. Other 

 species occur in Rhodesia and the Indian rivers. 



Limnocoditim was first known from the Victoria Regia tank in the 

 Royal Botanic Gardens, but has now been discovered in various North 

 American rivers and has even colonized ponds and canals in England. 

 It has a polyp-like stage, Microhydra, which has a certain likeness to 

 Hydra. 



Order H YDROCORALLINAE 

 The forms included in this group are mostly associated with reef 

 corals in tropical seas. The main part of the colony consists of a 

 much branched hydrorhiza with frequent anastomoses. Instead of 

 secreting a horny perisarc as the Calyptoblastea and the Gymno- 

 blastea do, the ectoderm lays down an exoskeleton consisting of 

 calcareous grains, which becomes bulky and solid. It may be either 

 massive or encrusting or branching. From pits in the surface of the 

 colony arise the polyps. These are of two types (Fig. 120). First 

 there are the individuals of normal structure with a mouth sur- 

 rounded by tentacles (gasterozooids) : these nourish the colony. Then 

 there are the dactylozooids which are much longer and more slender. 

 They have no mouth but they possess scattered capitate tentacles and 

 may form a ring round a gasterozooid, in which case it is readily 

 observed that their function is to catch prey and hand it to the central 

 gasterozooid for digestion. Besides the polyps there are the medu- 

 soids, which, as in Bougainvillea, are budded directly off from the 



