THE MOST PRIMITIVE METAZOA 85 



(14) Gastrodes has a planula larva. 



(15) Certain coelenterates such as Hydroctena resemble 

 ctenophores. 



Differences between Coelenterata and Ctenophora 



(1) Some ctenophores have openings to the gut other than the 

 mouth. (Similar openings are found in some medusae such 

 as Aequorea.) 



(2) The lasso cells are morphologically quite distinct from 

 nematocysts. The nematocysts of Enchlora have been 

 stated to have been derived from its food. 



(3) There are no musculo-epithelial cells. 



(4) Coeloplana and Ctenoplana have genital ducts. 



(5) The cilia are arranged in specific rows, or comb plates 

 (except for Coeloplana). 



(6) Their symmetry is more biradial than radial. 



(7) The embryology of the ctenophores is determinate and the 

 cleavage differs markedly from that of the coelenterates. 



Can the Ctenophora be placed in the Coelenterata? It can 



be seen that there are many resemblances between the coelenter- 

 ates and the ctenophores. The two greatest differences seem to lie 

 in the possession of nematocysts by the coelenterates and the 

 embryology of the two groups. The ctenophores do possess lasso 

 cells which differ in their morphology from the nematocysts found 

 in the coelenterates. However, one ctenophore, Euchlora rubra, 

 has been found to have nematocysts. When these were investi- 

 gated by Komai (1942) he found that the nematocysts occurred 

 in the tentacles and inside cells which he thought were endodermal 

 cells. In 1951 Komai suggested that the nematocysts might have 

 been taken from the food of Euchlora since the nematocysts did not 

 lie on the surface of the tentacle but were sunk into the ectoderm. 

 Hadzi (1951) thought that the nematocysts in Euchlora were 

 derived from its food, there being a close resemblance between its 

 nematocysts and those of the narcomedusan Cunina. 



Picard (1955) reinvestigated this problem and found that the 

 nematocysts did in fact lie on the surface of the ectoderm, correctly 

 orientated for discharge. The nematocysts were only found in 

 association with ectodermal cells, never endodermal cells. Picard 



