34.2 NOTES AND M EMOHAN D A. 



pi'dota, are absent, according to the ILrLvvigs, tliougli stated 

 to be present by Glaus. The peripheral nervons system has 

 been especially studied by Scliufer, and its presence is mentioned 

 by Glaus. Schiifer describes it as formed of an interlacement of 

 nerve-fibres covering the whole under surfaci; of the umbrella and 

 lying between the ectodernal epithelium and the muscular sheet. 

 Each nerve-fibre ])rcsents in the middle of its course a nucleated 

 enlargement in the shape of a bipolar nerve-cell, which is thus 

 interpolated in the course of the fibre. The nerve-fibres are 

 rarely more than four millimetres in length, and do not come into 

 actual continuity with other fibres. They end either by fine 

 tapering extremities, or by dilated expansions enclosing a nucleus. 

 Tiie nucleated expansions are regarded by Schiifer as a form of 

 motorial end plate. It will be seen that that the peripheral 

 nervous system of the Acraspeda appears to difier nearly as much 

 from the same system in Graspedota as does the central system. 



In considering the organs of special sense, it will be convenient 

 again to distinguish between the Graspedota and the Acraspeda. 

 In the Graspedota we are entirely indebted to the brothers 

 Hertwig for our facts. 



Though both auditory and optical organs are present in this 

 group, they are never associated in a single form. Organs of 

 hearing are formed in the Trachymedusse and the Vesiculata, 

 and, as the brothers Hertwig clearly prove, are formed on different 

 types in these two groups. 



In the Vesiculata the simplest form of auditory organ is that of a 

 series of open pits situated along the attaclied edge of the velum 

 with the aperture directed downwards. Both e[)ithelial layers of 

 the velum take part in their formation. The epithelium of the 

 upper surface of the velum covers the convex surface of the 

 organs, and its cells are provided with thick membranes, and 

 filled with fiuid. The epithelium of the under side of the velum 

 supplies the cells of the under or concave side of the organ, and 

 most of it cells develope a calcarious concretion ; but a row of 

 them along the inner edge of each pit, takes the form of sense- 

 cells, provided with auditory hairs, and continuous with the 

 fibres of the lower nerve ring. The above form of auditory 

 organ is found in Mitrotrocha, Tiaropsis and other genera. In 

 Mitrotocha the number of such pits present in a single individual 

 may amount to eighty. 



In many Vesiculata (Acquorea Octorchis Euchcilota, etc.) the 

 open pits are replaced by closed sacks. The open and closed 

 forms of the organ ai)pear at first sight rather different, but really 

 stand to each other in the same relation as do the open auditory 



