158 BULLETIN OF THE 



fibres which are attached to the proximal side of the basement membrane 

 being, in refractive power and other optical properties, and in the char- 

 acter of their nuclei, indistinguishable from the other cutis fibres. One 

 often finds such an appearance as is given in Figure 16. This is evi- 

 dently a tangential section of a similar organ ; the central clear space 

 represents the hollow produced by the retracted apical area, and the 

 apparently round nuclei are merely the elongated forms transsected. The 

 appearance of the cells suggests no glandular nature, and nothing could 

 be found resembling a secretion. For this reason I am inclined to ques- 

 tion the propriety of the name " Wimperdriisen " (Vogt und Yung, '88, 

 p. 40G), and to regard them as simple sensory organs. The retraction of 

 the apical area would then be a simple method for protecting the long and 

 delicate cilia during the advance of the animal through sand, similar to 

 that reported by other observers for such organs in various groups. I was 

 unable to discover any nerves connected with these organs, so that their 

 sensory nature remains unproved, although none the less probable (Eisig, 

 '87, p. 548). The structures just described are distributed over the 

 aboral surface of the tentacles in somewhat irregular lines, becoming less 

 frequent toward the margin of the fold, but are not present on its oral 

 aspect. They suggest strongly the diffuse sensory organs (Becherorgane) 

 of Capitellida?, described by Eisig ('87, p. 547), but they are certainly 

 less highly differentiated in the following respects : — 1. The cilia are not 

 confined to the apical area (Polfeld), but are more or less diffused over 

 the entire prominence. 2. There are only a few of the nervous nuclei 

 (Korner) present in the basal portion. These structures recall the cup- 

 shaped organs of CapiteUida) most strongly in the character of their ele- 

 ments, the filamentous cells, in their relation to the general hypodermis, 

 and in the thin cuticula which covers them. In both cases, connection 

 with nervous elements remains a matter of conjecture. 



Very similar organs have been described by Spengel ('80, p. 465) for 

 Echiurus, as appears at once from a comparison of the figures given by 

 that author (Taf. XXIV. Figs. 21, 22). These, however, differ materi- 

 ally from those in Sipunculus in two respects : first, no cilia were present 

 (Spengel believes them to have been lost through poor preservation); sec- 

 ondly, a fact of more importance, a large number of unicellular glands are 

 found immediately below and in connection with these organs in Echiu- 

 rus. The latter are certainly not present in Sipunculus. The distribu- 

 tion of these organs is quite different in the two forms, since there occur 

 from one to seven on each of the papillse of Echiurus, whereas in Si- 

 punculus they are confined to the small anterior zone of the introvert. 



