ON LARViE OF LINGULA AND PELAGODISCUS (DISCINISCA). 51 



and is Lent upon itself near the middle of its length, so that the terminal part is 

 dorsal and its tip directed to the left side. 



The other organs are similar to those of the preceding stage, except that the two 

 ccelomoducts (nephridia) are now very clearly seen (fig. 10). 



Three of the four larger larvse of this group have been cut into serial sections, 

 and a few notes on these may he given here (see figs. 8, 9, 10). 



The shell was found to be in most parts about 20/x thick,* increasing to 30/x near 

 the posterior margin, and to consist of a chitinoid substance, a thin outer layer of 

 which forms a not very clearly differentiated periostracum. 



The statocysts (fig. 8) are closed sacs the external antero-posterior and transverse 

 diameters of which are about 40-50/x, and the dorso-ventral diameter 10-1 5/x. 

 Several minute statoliths in motion f were observed in each statocyst in the living 

 specimens, but are not now visible. Each statocyst is situated in the dorsal body- 

 wall at the point where the latter is joined by the lateral body-wall, and the 

 appearances suggest that the statocyst was formed by an invagination of the 

 epithelium in the lateral angle of contact of the two portions of the body- wall, but 

 the statocyst now exhibits no trace of connection with the exterior. 



The three ganglia of the central nervous system are situated in the body- wall ; 

 the ventral or sub-oesophageal (fig. 10) is much larger than the two lateral ganglia, 

 which lie immediately in front of the anterior occlusors. 



Muscle-fibres, almost longitudinal in direction, have appeared in the peduncle 

 between the outer and inner cell-layers, but they are really formed in the latter. 



The median tentacle bears long cilia at its tip on the ventral side. The tentacle 

 is hollow, and the lumen,! which may not extend far into the distal half, contains 

 cells and muscle fibres, the latter serving to bend or contract the tentacle. Sur- 

 rounding the lumen in a zone of supporting substance of homogeneous nature, and 

 around this is a layer, apparently nervous, which is in contact with the base of the 

 sensory external epithelium. 



Each cirrus is also hollow, and the lumen contains a bundle of longitudinal muscle- 

 fibres which lie chiefly towards the medial side of the cirrus. The thick, external 

 epithelium, which is ciliated on its median aspect, has very deeply staining nuclei. 

 In the base of each cirrus there is homogeneous supporting substance, usually 

 thinner or absent on the median aspect, which is continued into the two arms of 

 the lophophore, where it forms, just proximal to the insertions of the cirri, a 

 continuous curved band (fig. 9) supporting this basal portion of the arm-apparatus. 

 The right and left bands are linked together by a small mass of the same kind of 

 supporting substance situated in the mid-ventral wall of the lophophore near its 



* The shells of these larvse with 13 pairs of cirri are about twice as thick as those figured by Dr Yatsu from 

 his larvse with 15 pairs of cirri. It may be noted that his larvse, from the stage with 7-9 pairs of cirri onwards, 

 were reared in captivity. 



t Ciliated cells were not observed in the wall of the statocyst. 



t The lumen opens posteriorly into the arm-sinus, which is connected with the general body-cavity. 



