WYVILLE THOMSON, ON SYNAPTA INH^RENS. 135 



In Synapta inltarens I have observed no organs of special 

 sense. Miiller describes twelve eye-spots of the usual 

 form between the tentacles on the oral disc of S. digitata, 

 and in the young of H. inlKsrens vesicles will be described 

 which must, I should imagine from their structure and 

 position, be rudimentary organs of hearing; but I have heen 

 unable to detect the slightest trace even of these in the 

 mature form. The sarcode covering of the intestine, of the 

 ambulacral vessels, and of the reproductive tubes is richly 

 ciliated. 



When first observed, the young was only about "8 mm. in 

 length, and "6 mm. in diameter. It had the form of a 

 minute rounded sac, of glassy transparency, slightly con- 

 stricted towards the upper extremity, and above the constric- 

 tion a rudely vandyked frill indicated the rudiments of the 

 five primitive tentacles. PL V, fig. 1, represents the young 

 at this stage, slightly compressed, and viewed by transmitted 

 light. The internal organs are seen perfectly, through the 

 transparent outer envelope. The outer wall of the sac is 

 very thick, and formed of transparent sarcode. Under 

 ordinary powers the sarcode appears perfectly structureless, 

 but with a high power extremely minute spaces are seen 

 scattered through its substance, and apparently connected 

 by excessively delicate transparent threads or canals, the 

 whole structure resembling somewhat the lacunae and canali- 

 culi of bone. Groups of very minute greyish granules are 

 observed imbedded in the sarcode, and, scattered over the 

 surface are large lenticular cells, '03 mm, in diameter, filled 

 with a fluid with a low refractive index, and slightly brown by 

 transmitted light. I am inclined to regard these cells, which 

 are permanent during all the stages of the development of 

 the young, and which seem to be specially abundant in the 

 neighbourhood of developing calcareous plates, as reservoirs 

 of calcareous solution ; I cannot imagine, however, that they 

 correspond with the calcareous glands of Miiller. Close to 

 the posterior extremity there is a small group of calcareous 

 granules coloured yelloAvish by organic matter. Some of 

 these concretions seem to be irregular rhombic crystals, but 

 the prevailing form is a rosette of irregular wedges. These 

 granules increase for some time in number ; they disappear, 

 however, entirely, when the young has attained a certain 

 stage in development. 



Five glistening longitudinal muscular bands, as yet of ex- 

 treme tenuity, traverse the sarcode sac from the oesophageal 

 ring to the anal extremity. The transverse muscular fibres 

 cannot as yet be traced. A continuation of the outer sarcode 



