MYZOSTOMIDA FROM THE RED SEA. 



91 



spherical or oval bodies with walls so thick that the cavity 

 is almost obliterated or reduced to an irregular ramifying slit 

 between the folds of the wall. 



Text-fiff. 18. 



.A^\ 



Mj/sostoma costatum. — Sketch of an abnormal specimen 1'25 mm. in length. Onlj- 

 two paiapodia are present on one side of the bod j', and four on the other; the 

 number of suckers is normal. 



The number of suckers in Mr. Crossland's specimens is quite 

 constant, and, apart from some otherwise abnormal individuals, 

 there were only a few in which all twelve could not be made out 

 by meg,ns of the microscope. This is by far the most serious dis- 

 ci-epancy between my account of M. costatum and that of previous 

 authors, and for some time I considered establishing a new species 

 to receive Mr. Crossland's specimens. It seems probable, how- 

 ever, that the extra pairs of suckers have been overlooked ; even 

 when well developed these organs are difficult to make out on 

 unmounted specimens of Myzostomids, a fact which the majority 

 of workers on this group have commented on from time to time. 

 Leuckart's description of M. costatum was naturally incomplete, 

 and the few specimens seen by v. Graff seem to have been in a 

 rather poor state of preservation and much contracted ; no mounts 

 were made, and under the circumstances the extra suckers, if 

 present, might easily have remained unnoticed. 



The presence of six pairs of suckers in a species of Myzostoma 

 appears of considerable interest, since it necessitates some modifi- 

 cation in the definition of the group, as Avell as of the generally 



