266 GILBERT C. BOUimK. 



that they are iii some inauuer counocted with the insertions 

 of the muscle-bands both of the branchial sphincter and the 

 longitudinal muscles of the body-wall, and tliat their thickened 

 walls afford support to the tissues of this region. 



It is clear from the foregoing description that Oligotrema, 

 aberrant as it is in many structural features, must be classed 

 in the family Molgulidas, for the following reasons : 



1. The arenaceous covering to the test and hollow, hair-like, 

 villous processes of the latter, serving for the attachment 

 of sand-grains, though found in some Cynthiadae and other 

 Ascidians, are features which on the whole are characteristic 

 of the Molgulid^e. 



2. The six branchial arms must be regarded as the highly 

 differentiated representatives of six branchial lobes. The 

 Molgulidas are characterised by the possession of six branchial 

 lobes, and in the genus Ctenicella, a member of this family, 

 the lobes are produced into short, muscular, tentacle-like pro- 

 cesses provided with terminal and lateral piunse comparable 

 to the arms of Oligotrema, though very much less developed. 

 The Molgulidas are further characterised by the presence of 

 four lobes surrounding the atrial siphon, and in Ctenicella 

 these are modified in the same manner as the branchial lobes. 

 There are no atrial lobes in Oligotrema, but their absence is 

 clearly attributable to the great reduction of the atrial aperture. 



3. The oral tentacles are compound, as is the case in all 

 Molgulida). 



4. The branchial sac is longitudinally folded, and the 

 representatives of the longitudinal bars are not papillated. 



5. The stigmata, though not arranged in spirals, are 

 slightly curved. The stigmata of Oligotrema, indeed, do not 

 show any marked resemblance to those of typical members of 

 the family, but it must be remembered that the spiral arrang'e- 

 ment does occur in all Molgulid;e. 



6. The intestine is on the left side of the stomach, and 

 throughout its course is embedded in the ectoplerome ; or, to 

 use the terminology of previous authors, it is attached to the 



