630 SUMMARY OF CURRENT RESEARCHES RELATING TO 



enterocoel, and that its connection with the hydroccel, therefrom 

 developed, is only a secondary phenomenon; this earlier relation is 

 retained by the Crinoidea, where the primary pore opens into the 

 enterocoel. After discussing the further development of the hydro- 

 ccel, and the history of the blood-vascular system and permanent 

 stomodseura, the author passes to a description of the external form of 

 the larva, where attention is directed to the so-called " larval organ," 

 or two cephalic lobes, the one smaller and anterior, the other larger 

 and posterior, which form a special locomotor organ and are at the 

 height of their development from the seventh to the ninth days. The 

 wall of this organ consists of the three layers of the body, fine 

 muscular fibres being developed on the outer side of the endodermal 

 layer ; the cavity of the organ is a development of the enterocoel, and 

 the whole structure may be looked upon as being the homologue of 

 the arms of a Srachiolaria. 



In an account of the development of the skeleton it is necessary 

 to distinguish two groups of skeletal parts, both of which appear 

 early ; to one are due the rudiments of the ambulacral pieces of the 

 future arms, and to the other those of the primary skeletal pieces of 

 the dorsal side of the starfish. A detailed study of the former shows 

 that they arise in just the same way as in Ophiurans ; the latter are 

 developed in the mesoderm of the body wall, and on the seventh day 

 eleven pieces may be made out ; one of these always has a remarkably 

 constant relation to the dorsal pores, and is the one which becomes 

 converted into the madreporic plate ; the pore does not primitively 

 lie in the plate, but to the left of it ; this relation is of significance 

 when compared with what is seen in other Echinoderms ; in Ophiurans 

 the pore lies towards the left margin of the plate, and in Crinoids the 

 primary pore has a similar relation to one of the oral plates. The 

 other ten pieces do not all arise together, or exhibit the same rate of 

 growth ; five of them are the rudiments of the so-called " radialia," or 

 better, " terminalia " of the arms of the starfish ; within and alternat- 

 ing with them lie the primary interradial plates, one of which forms 

 the madreporite ; the eleventh piece occupies a more central position 

 and is the rudiment of the central plate of the back. 



As the larva derived from the gastrula gradually passes into the 

 young sea-star, the larval organs become lost, the " larval organ " and 

 the stomodseum of the larva being the last to be retained. On the 

 ninth day, however, the larval organ decreases in size, till on the 

 tenth or eleventh it forms nothing but a short stalked and knobbed 

 process. This is doubtless the peculiar peduncle noted by Desor in 

 a species of Echinaster, and corresponds to the different structures to 

 which L. Agassiz, Wyv. Thomson, and Philippi have directed atten- 

 tion. After describing the history of the enteric tract, and especially 

 the mouth, the author raises the question of whether there is a plane 

 of symmetry in the young corresponding to the median plane of the 

 larva ; he thinks that none is to be, or indeed can be, detected ; want 

 of symmetry is one of the leading characters in Echinoderm structure ; 

 yet this asymmetry is ordered by definite laws. If any radius or 

 interradius is to be regarded as the "anterior" one, it must be that 



