3 g 4 DISCOVERY REPORTS 



Thin strips of ectoderm are hard to obtain because of the deep folding of the layer. It would be of 

 great interest to obtain histological data about the nerves in these regions, because Parker's finding of 

 a high conduction rate (120 cm./sec. at 26 C.) makes it probable that the neurons would show a 

 longitudinal orientation as they do in the mesenteries of Calliactis (Pantin, 1952). 



glen, -* 



elenteron 



gl ec. 3 



50,u 



Text-fig. 3. Transverse section through adjacent portions of the codon (C) and saccus (S). br = bridge cell, c 1 = cuticle 

 of codon, c- = pneumatocyst, gl.ec 1 = ectodermal gland cell, gl.ec 2 = root-like cuticular body believed to represent site of 

 extinct gland cell, gl.ec 3 = similar body attaching pneumatocyst, gl.en = endodermal gland cell opening into the coelenteron, 

 mes = mesogloea, m.ec, m.en = muscle fibres of ectoderm and endoderm respectively, netn = nematocyst, \n = tetraploid 

 nucleus, s.n = nerve-cell with sense hair. 



An examination of the sexual medusoids (gonophores) from formol-fixed 'Discovery' material 

 revealed the presence of nerves in the ectoderm of both ex- and subumbrellar surfaces. These nerves 

 have been overlooked by previous workers using the sectioning technique (for example Steche, 1907). 

 In the present investigation, they were studied in strips of tissue peeled off from the mesogloea, and 

 stained in Hansen's trioxyhaematein. The nerve-cells are distributed sparsely but evenly. A point 

 of interest is that there is no differentiation of the plexus into marginal nerve-rings. In medusae and 

 swimming-bells where rhythmic pulsations take place, two marginal nerve-rings, one subumbrellar 

 and one exumbrellar, are found in the ectoderm at the base of the velum. In Physalia the development 

 of the gonophore is halted in the stage known as 'eumedusoid' (Hyman, 1940); the gonophore 

 probably has no capacity for movement, judging from the almost complete absence of muscle fibres 

 from the subumbrellar. One assumes that the ancestor of Physalia had free-swimming gonophores 

 and that the captive, reduced condition is a secondary one. But it is not easy to say whether the 

 diffuse, undifferentiated arrangement of the nervous system is degenerate, or whether it represents the 

 ancestral pattern, or only a stage in the development from the ancestral pattern. At all events, it 

 represents a hypothetical ancestral condition, from which the nervous systems of all hydrozoan 

 medusae and medusoids can be held to derive. The chief specialization of the original diffuse plexus 

 has, of course, been the development of the marginal rings, but evolution has led also to the complete 

 loss of the plexus from certain regions (for example, the subumbrella of swimming-bells) and its 

 elaboration into exumbrellar tracts (for example, in the swimming-bells of certain Physonectae). It 

 may one day become possible to give a complete and coherent explanation for these developments, in 

 terms both of form and function. 



