424 Dr. II. Luclwig on the 



Auditory organs, which from general considerations I hoped 

 to Hiid, I sought for entirely in vain. In no shape, and at no 

 stage of development, either upon the nerve-ring or the radial 

 nerves, was I able to deteet anything of tlie kind. 



The muscuhdure of the bodi/-iva II is furnished from tlie cells 

 of the parietal enteroecele. First to be formed is the median 

 ventral longitudinal muscle, which, on the ninth day, can 

 already be distinguished as a fine single layer of longitudinal 

 fibres on the inner side of the median ventral radial vessel. 

 On the thirteenth day the rudiment of this muscle has already 

 become somewhat broader than the transverse diameter of the 

 radial vessel. The separate fibres of which the muscle con- 

 sists lie closer together than the muscle-fibres in the outer 

 wall of the radial vessel, from which they are subsequently 

 still further distinguished by their more than double thick- 

 ness. In front the young longitudinal muscle commences (as 

 in the case of the adult animal) on the outer side of tlie corre- 

 sponding radial ossicle of the pharyngeal ring; posteriorly it 

 extends as far as the region of the origin of the first two 

 pedal vessels. 



Not until after tlie median ventral longitudinal muscle has 

 been formed do we observe, on the fifteenth day, isolated 

 tiaiisvcrse muscle-fibres on the outer surface of the parietal 

 enteroecele, and on the eighteenth day a transverse muscular 

 layer of the body-wall, interrupted in the radii, is distinctly 

 visible. At the anus the transverse muscle-fibres draw closer 

 together and form round it a sphincter-muscle (forty-fifth 



The four longitudinal muscles of the lateral radii in the 

 order of their appearance and in their original inequality of 

 strength follow the relations of the radial vessels and the 

 radial nerves, since in their case also the two latero-dorsal 

 precede the two latero-ventral ones both in point of time and 

 in actual length. The former'are visible on the seventeenth 

 day, the latter not until the forty-fifth. 



The splitting-off of the retractile muscles from the longi- 

 tudinal ones a})pears to take place very late, since I was only 

 able to detect the first traces of it in a few individuals of the 

 hundred and eleventh day. 



27ie calcareous bodies of the integument are already visible 

 in the stage of the barrel-shaped larva, and are taken over 

 en masse by the young Cucumaria, so that a true larval 

 skeleton, peculiar to the larva, does not exist. Each calca- 

 reous body originally has the form of a tiny rod, which, by 

 repeated bifurcations of its ends, which always take place at 

 an angle of 120°, and subsequently by tin' contact and fusiuu 



