416 Dr. II. Luclwig on the 



On the following days it becomes more and more evident that 

 this vessel exceeds the other four not only in length, but also 

 in diameter. But, in addition to this, tliese four differ again 

 among themselves, for the two latero-ventral canals are 

 shorter and narrower than the two latero-dorsal ones. This 

 difference between the five radial vessels continues far into 

 the life of the young animal, and is only adjusted at a late 

 period by means of subsequent processes of growth on the 

 part of the four lateral radial vessels. Again, with regard to 

 the formation of the musculature in thewallsof the radial vessels, 

 the median ventral canal is in advance of the remaining four, 

 andamong these, again, the.two dorsal ones are in advance of the 

 two ventral. For while the first distinct muscle-fibres appear 

 in the Avail of the median ventral radial vessel as early as the 

 thirteenth day, it is not until the seventeenth day that the 

 two latero-dorsal vessels acquire their first muscle-fibres, 

 while tiiree days more elapse before a similar event occurs for 

 the two latero-ventral radial vessels. The whole of these 

 muscle-fibres are limited to that section of the radial vessels 

 which lies externally and posteriorly to the ring of pharyn- 

 geal ossicles. On the other hand, in the short portions of 

 the radial vessels which lead to the water-vascular ring 

 internally to the radial pharyngeal ossicles, I was still unable 

 to detect any trace of muscle-fibres on the forty-fifth day of 

 development. The muscle-fibres of the radial vessels are all 

 longitudinal, are supplied from the cells of the epithelium of 

 the hydrocoele, and occur {as in the case of the adult animal) 

 in that wall only of the radial vessels which is turned towards 

 the upper surface of the body, where tliey are arranged side 

 by side to form a single layer. 



The relations in which the young tentacles stand towards 

 the regions of the body and the water-vascular system prove 

 to be of especial interest. On the eighth day of development 

 five tentacles have already been developed. Their position 

 -with regard to the mouth, and particularly with regard to the 

 ciliated bands of the barrel-siiaped larva, is different from 

 that described by Selenka. They lie in a spacious oral atrium, 

 into which they can be completely retracted ; tlie atrium is 

 tiien connected with the exterior by means of a circular sharp- 

 edged opening. If, however, the tentacles are extended the 

 oral atrium simultaneously flattens out, and the tentacles now 

 enable it to be seen that they are all five situated in front of 

 the second ciliated band of the larva (I regard the cilia of the 

 ceplialic hump as the first ciliated band). Seleidca further 

 states that the first five tentacles, when they are extended, 

 are so arranged t!iat, commencing from the front, Ave can 



