464 SUMMARY OF CL'EKENT KESEARCHES RELATING TO 



and of radial svmmetrv svith sedentary or floatins; life, and collectinir 

 food from any direction. Meristic varialnlity in important organs is 

 much greater in radially symmetrical forms than in bilaterally 

 symmetrical forms where a balance must be kept. In sedentary 

 colonies plasticity of shape is of great vital importance. " With this 

 great plasticity in the method of growth, and therefore the great 

 accommodation of shape to the conditions of the environment, many of 

 the characters used by systematists for the distinction of species must 

 lie regarded as of very little value." In reference to the Pennatulacea 

 Hickson shows how variable or plastic characters may become less 

 variable or plastic as a transition is made from radial to l)ilateral 

 symmetry, and how the increasing rigidity of certain characters leads in 

 some cases to the differentiation of the discontinuous groups which are 

 recognized as species. 



Larval and Post-larval Fishes of 'Terra Nova' Expedition.* — 

 C. Tate Regan reports on a post-larval example of the strictly Antarctic 

 Notolepis coatsii Dollo, and on a series illustrating the development of 

 the more widely distributed Myctophum antarctkum. Giinth. The 

 larval and post-larval specimens of the latter were all taken in the 

 Sulmntarctic zone ; the adult ranges throughout the Antarctic, Sub- 

 antarctic and South Temperate zones. A second Antarctic species of 

 Paraliparis is reported, and named P. terrse-norae sp. n. The collection 

 also included young stages of some Nototheuiiform fishes — Pleura- 

 gramma, Pagetop^is, and Ghionodraco. A number of fishes collected 

 from Tropical and South Temperate zones are also dealt with. 



It is well known that the truth of the theory that ontogeny repeats 

 phylogeny is shown by almost every Teleostean fish in the development 

 of its caudal fin, which is at first ventral and then becomes terminal. 

 Two rather puzzling developmental features, the migration forwards of 

 the dorsal fin in the Clupeida and the migration backwards of the anus 

 in Notolepis, Paralepis, etc., may possibly be explained ])y the same 

 theory. This is discussed. 



The pectoral fins, which are principally concerned with balancing, 

 are usually present in the youngest larvte, and the permanent fin-rays 

 appear at a very early stage. As a rule the caudal, used for propulsion, 

 is the next fin to develop. The hypurals and fin-rays make their 

 appearance below the notochord, and then l)y flexion of the latter are 

 brought into a terminal position. Afterwards the dorsal and anal rays 

 develop in the embryonic fin-fold and the pelvic fins grow out. The 

 deviations from this, the usual, order are considered. 



Pelagic larvae are characterized by the absence of accessory organs 

 of respiration and of adhesive organs ; by their invisibility ; by their 

 Imoyancy and balance ; by their well-developed sense-organs ; and l)y 

 the frequently defensive bones of the head, which are armed with spines. 



Except in a few groups, e.g. Apodes, there are no features that 

 characterize pelagic larval fishes as belonging to one order or another, 



* British Antarctic (' Terra Nova ') Expedition (Zool.) i. (1916) No. 4, pp. 

 125-56 (10 pis. and 5 figs.). 



