574 SUMMARY OF CURRENT RESEARCHES RELATING TO 



api^arent enough. The blastula is of the same thickness throughout ; 

 in Echinids, and probably also in Asterids and Ophiurids the blasto- 

 dermic cells are broader on the lower surface, in the Holothuroidea 

 they are of the same size all round. The mesoblast arises from the 

 primitive cells of the mesenchym and from the diverticula of the 

 archenteron. The former, by means of their daughter-cells, and in 

 the form of wandering cells, make their way into the blastocoelom and 

 give rise to the circular musculature of the fore-gut and to the cutis. 

 The archenteric diverticula and their derivates consist first of a 

 single layer of cells, from which later on scattered cells arise peri- 

 pherally and form an outer ring of unicellular muscles. The 

 explanation of this double mode of origin is not easy ; it may be 

 said that the two primitive cells of the mesenchym are the homologues 

 of the two primitive cells of the mesoblast of molluscs, Arthropods, 

 &c., and that the archenteric diverticula are new formations ("neo- 

 morphs ") ; while there are several good reasons to be given in support 

 of this hypothesis there are others that favour the view that the 

 diverticula form the primitive seat of origin of the mesoblast and 

 that the mesenchymatous cells are cenogenetic. Lastly, and this is 

 perhaps the best view of all, the mesenchym-cells are portions of the 

 archenteric diverticula, which in consequence of the modification of 

 the larval life, have precociously separated from the rest. 



The " blood-corpuscles " of the water- vessels are found to arise from 

 epithelial cells of the rudiments of the water-vessels, and those of the 

 enterocoelom from the peritoneal or coelomic epithelium. 



Evidence of the vermian origin of Echinoderms is afforded by the 

 primary mesoderm having the form of two primitive cells, and by 

 the bilateral symmetry of the larval organs. The division of the 

 archenteric diverticulum into coelomic sac and water-vessels corre- 

 sponds physiologically to that which the mesodermic sac undergoes in 

 Vertebrates, and to some extent in worms. 



New Genus of Echinoids.*— Prof. F. Jeffrey Bell institutes a new 

 genus for the Echinanthus tumidus described a few years since by 

 Mr. Tenison- Woods, on the ground that the rows of ambulacral pores, 

 instead of being approximated at their free end, tend to widen out in 

 a lyre-shaped fashion ; and the genus is thereby removed " from the 

 direct line of ancestry through which the ortliostichous passed to the 

 petalosticlwus Echinids." He alludes to the significance of this form 

 being found in the Australian seas, and expresses a belief that further 

 research will result in the discovery of other forms which have been 

 unsuccessful in the struggle for existence. 



Revision of the Genus Oreaster.f — Prof. F. Jeffrey Bell revises 

 the twenty-seven known species of Oreaster and describes five new 

 species ; in the systematic disposal of the species he has attempted to 

 gain some assistance from the study of their post-larval development, 

 especially as reggjrds the number and arrangement of their spines ; he 



* Proc. Zool. Soc. Loud., 1884, pp. 40-1 (2 pis.). 

 t Tom. cit., pp. 57-87. 



