48 Nils Hj. Odhner. 
region the body walls show their maximum degree of flat- 
ness or concavity (fig. 74). In the ventral musculature we observe, 
as described by PRuvor and THIELE, a strong cord of longitudinal 
muscles separated from the remaining longitudinal layer, which is 
thickened on both sides of the ventral furrow and thence tapers 
upwards. 
Outside the roots of the transversal muscles, which are sur- 
rounded by the cells of the pedal glands, run the two pedal nerve 
cords and the lateral nerves somewhat below the middle of the 
body inside the musculature. 
The dorsal sensory organ appears as a wart-like projection 
on the dorsal side just in front of the cloacal cavity (fig. 71). It is 
composed of a central tissue and a superficial covering of epithelial 
cells. The apex of the wart is covered with a rather thin cuticula. 
Beneath it a spherical mass of slender cells appears; each of the, 
cells has a mesially placed nucleus and the base produced into a 
thin thread descending along the axis of the organ. The central 
portion is filled with a mass of bladder-like cells ptobably of 
ganglious nature, among which the basal threads of the sensory 
cells disappear. Also on the sides of the warty projection sensory 
cells of the same shape occur, intermixed with the epithelial ones. 
The circular muscular layer follows the overlying epidermis into 
the base of the papilla, where it is pierced by the central nerve 
tissue. The longitudinal muscles, however, form a continuous 
layer beneath the nervous tissue and are followed on their inside 
by the great dorsal blood lacuna. 
Fam. Gymnomeniidae n. fam. 
Cuticula thin, epidermis a simple multicellular layer without 
papillae; no spicula. 
Gymnomenia n. gen. 
Body rather short and thick, rounded square in section, with 
a convex upper side, somewhat concave lateral sides and flattened 
ventrally; anterior end truncated, posterior one shortly acuminated. 
Ventral side with a median furrow flanked with a broad swelling at 
each side and containing one fold terminating in a posterior papilla 
