136 WYVILLE THOMSON, ON SYNAPTA INH^REXS. 



layer forms the tentacular fringe^ and round the end of each 

 tentacle there is a thick border of pyriform cells imbedded 

 in tlie sarcode. The rudimentary tentacles move actively, 

 the animal uses them alone, and indiflferently, as organs of 

 progression, and they adhere firmly even to a surface of 

 smooth glass. 



Ten minute symmetrical calcareous spiculae form a ring 

 rovnid the oesophagus. Each spicula consists of a short 

 trunk, dividing at one end into two branches, which in some 

 cases divide again. The spiculae are arranged in pairs, each 

 undivided end turned towards the simple end of the spicula 

 next it. Immediately within this ring the circular canal of 

 the ambulacral vascular system forms a delicate annular 

 lacuna in the transparent sarcode. Five short csecal pro- 

 cesses pass towards the salient angles of the tentacular frill, 

 and a finely granular discoid mass, nearly sessile on the lower 

 surface of the ring, indicates the madreporic tubercle. The 

 five delicate ambulacral branches can already be detected 

 from their separating the longitudinal muscular bands into 

 two bundles, but there seems to be no trace of a Poliau 

 vesicle. 



At this early stage ten vesicles, one on either side of each 

 of the longitudinal muscles, close to its origin in the cal- 

 careous ring, are remarkably prominent. These vesicles are 

 oval, "025 mm. in long diametei', and contain large secondary 

 vesicles, containing in turn large vesicular nuclei ; viewed by 

 light transmitted obliquely in certain directions, the contents 

 of the outer and inner vesicles are dark, the intermediate cell 

 appearing as a bright ring, while by a change in the direc- 

 tion of the light the middle vesicle is darkened, and the 

 outer and inner brightly illuminated, leaving it in doubt 

 whether the transparent colourless fluid contents of the 

 several vesicles have the same or different refractive indices. 



I have little doubt that the oval vesicles are organs of 

 special sense, probably rudimentary organs of hearing. They 

 evidently correspond with the vesicles with double vibrating 

 nuclei, described by Miiller in young Holothuriae, and with 

 the marginal vesicles of the Hydromedusae. 



The mouth is a simple orifice in the centre of the tentacular 

 ring, and a delicate, slightly curved alimentary canal ends 

 in an obscure anal pore at the opposite extremity. The 

 middle of the alimentary canal is dilated into a distinct 

 stomach cavity, and its walls are granular, indicating a dis- 

 tinct differentiation of a special assimilating surface; but the 

 alimentary canal is perfectly motionless, and there appears to 

 be no free space between the sarcode of the body wall and 



