141 ^VYV^LLE Thomson, on synapta inh^rens. 



from the mouth, a minute umbilicoid depression, punctured 

 by several Aery small apertures, might be detected. 



From this point a delicate tube arose, encircled near its 

 origin by a branched calcareous ring, and continued into a 

 circular water-canal round the oesophagus. On the 18th of 

 INIarch the cilia had disappeared, and the embryos had sunk 

 to the bottom of the jar. Five cseca from the circular canal 

 formed a row of rudimentary tentacles round the mouth, and 

 several branched calcareous plates indicated the position of 

 the calcareous oesophageal ring. 



Oa the 25th of March the tentacles had become furnished 

 with minute marginal sucking papillte, and by their means 

 the embryo crawled on the surface of the glass. Two large 

 hollows, the first trace of the ambulacral feet, were perceptible 

 on the ventral surface near the posterior extremity. On the 

 circular canal between each pair of tentacles there was ob- 

 served a minute round vesicle. These vesicles the authors 

 regard as the origin of the second series of tentacles ; their 

 figure, however (op. cit., pi. vii, fig. 14, a), certainly repre- 

 sents them as nucleated vesicles, closely resembling in posi- 

 tion and appearance the vesicles which I have regarded in the 

 corresponding stage of Synapta, as rudimentary organs of 

 special sense. 



Five delicate longitudinal canals might now be traced from 

 the circular canal to the posterior extremitj' of the body, and 

 from one of these, two lateral branches ended in the ampullae 

 of the two water-feet. A short, narrow intestine passed from 

 the stomach to the excretory cloaca. 



On the 28th of jMarch the tentacles were still more fully 

 developed, and calcareous plates were found in their walls. 

 The feet were very apparent on the ventral surface; tlie 

 Polian vesicle Avas fully formed ; and the tube described above 

 as connecting the dorsal umbilicus with the water-canal had 

 separated from, the outer wall, and hung free in the body 

 cavity, its free portion, the rudimentary sand-canal, filled with 

 loose calcareous rods. The external integument was rapidly 

 becoming opaque, from the development of a network of 

 cribriform, calcareous plates. 



During the month of April the development of all the 

 essential characters of the genus Holothuria, advanced steadily 

 with certain special peculiarities, which referred the young, 

 without doubt, to a well-known Norwegian species — H. 

 tremuJa. On the 6th May, from a neglect of some necessary 

 precautions, the whole brood died. The authors observe 

 specially that towards the close of the period during which 



