188 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Pentaceros retindatus shows similar sensitiveness in the skin-gills and 

 oral tube-feet when a shadow or a bright spot is cast on them ; but the 

 spines and pedicellarije do not. 



If a healthy Toxop/ieustes is placed in a dish of water exposed to 

 direct sunlight, or even to diffuse light, and if then a patch of shadow 

 is suddenly cast upon its aboral surface, an almost instantaneous reaction 

 will occur in this region. The extended tube-feet react first ; then the 

 large pedicellariffi, whose jaws were wide open, close suddenly ; after 

 them the small pedicellariae react by opening up their jaws, and finally, 

 the spines bend towards the centre of stimulus. 



Cowles has also found that the pedicellariae react quite definitely to 

 a shadow even after they have been cut off. This confirms Uexkiill's 

 view that the pedicellarise have the dignity of a reflex person. They 

 can evidently react independently of the radial nerve. 



Evolution of Apical System of Holectypoida.* — H. L. Hawkins 

 has made a comparative study of the apical system, which he regards as 

 a phylogeuetic index of the first importance. He deals with Pygaater^ 

 Anorthopygus, HoJectypus, Discoklea, and Cotiulus, and indicates the 

 general trend of the evolution. 



Variation in Echinoid Plutei.f — David H. Tennent has studied 

 the variations in the skeleton of the larvte of Toxojmeustes variegatus 

 reared in the laboratory. He divides these variations into three classes : 

 fluctuating variations, defects, and multiplicities. He finds that there 

 are characteristic type or line variations. The development of the 

 embryos in an individual series shows a mean, which is not the mean of 

 the general population of echinoid plutei, and each series or line 

 exhibits its characteristic variations. 



Remarkable Pluteus. % — Caswell Grave describes a remarkable 

 pluteus of the sand-dollar MelUta penfapora discovered among a number 

 of preserved specimens. It possessed not only right and left hydro- 

 coeles, but well-developed ectodermic pouches on both sides. Moreover, 

 the right as well as the left posterior body cavity showed the changes 

 incident to the formation of the oral (hypogastric) coelom of the adult 

 echinoderm. In short, the bilateral symmetry of the pluteus was perfect 

 in every detail. A complete change in the fate and function of the 

 right posterior body-cavity was also involved. In normal development, 

 this body-cavity becomes transformed into the epigastric ccelom of the 

 adult sand-dollar ; in the abnormal pluteus, it was apparently destined 

 for transformation into the hypogastric coelome and its derivative, 

 among which the dentary apparatus is an important one. As possible 

 interpretations of the origin and development of redundant duplicate 

 structures on the right side of the pluteus, the author considers the 

 following suggestions : (a) that it is a reversion to type ; (b) that it is a 

 mutation or homoeotic variation ; (c) that it was due to some mistake in 

 the localization of the materials in the egg ; and (d) that it was due to 

 polyspermy. 



* Geol. :\rag., ix. (1912) pp. 8-16 (1 pi.). 



t Journ. Exper. ZooL, ix. (1911) pp. 657-714 (21 figs.). 



+ Johns Hopkins Univ. Circ, 1911, No. 2, pp. 35-46 (5 figs.) 



