84 THE VOYAGE OF H.M.S. CHALLENGER. 



Usually both openings are either four-lobed or cross-slit — that is, the lobes are triangular, 

 and fit together so closely as to reduce the apertures to four narrow slits, radiating from 

 a point so as to form a cross (PI. XX. fig. 1). In the genus Culeolus (belonging to the 

 sub-family Bolteninaj), however, the apertures have each less than four lobes, the branchial 

 being triangular and the atrial bilabiate. 



The shape of the body varies greatly, more than in any other family ; ranging from the 

 squat blister-like Styela grossularia to the long pedunculated Boltenia, and including a 

 number of irregular and curious forms such as the species of Microcosmus. 



The condition of the test is also variable, and can hardly be said to characterise the 

 family. In the majority of the species, however, and in the most typical forms (those 

 belonging to the Cynthinse), the test is leathery, comparatively thin, but tough aud 

 roughened on the outer surface. In some species {e.g., Boltenia pachydermatina, 

 Culeolus ivyville-thomsoni, &c.) it is thick and cartilaginous, while in a few remaining cases 

 (e.g., Culeolus perlucidus) it is thin and membranous. 



The mantle is very muscular in the majority of the Cynthiidse, and in its most highly 

 developed condition consists of three layers of muscle fibres — an outer longitudinal 

 and an inner longitudinal separated by a middle circular. In most cases, however, the 

 inner layer is absent, and there are only the two well-developed layers, the outer longi- 

 tudinal and the inner circular crossing at right angles. 



In Culeolus murrayi and most of the other Bolteninse they do not form continuous 

 layers, consequently the musculature has the appearance of an open network formed 

 by the longitudinal and circular bundles of fibres. 



In some Cynthiidse (e.g., Styela oblonga) the musculature is very feeble, and is re- 

 duced to a few faint longitudinal bundles, while in Styela flava (PI. XX. fig. 3) the 

 arrangement of the fibres appears to be quite irregular, and no trace of longitudinal and 

 circular layers can be made out. 



The branchial sac has longitudinal folds, and in typical members of the sub-family 

 Cynthinse they are very large, and sometimes numerous (at least twenty -four in Cynthia 

 grandis according to Heller). 1 In the Styelinaa the folds are reduced in number and 

 simplified. The typical number in this sub-family is eight, four on each side ; this number 

 is never exceeded, but is sometimes much reduced, as in Styela grossularia, van Beneden, 

 where there is only a single fold. 



In Culeolus and Fungidus the whole sac is much simplified by the absence of the 

 system of fine longitudinal or interstigmatic vessels. In all the other Cynthiidse the 

 stigmata are straight, and are arranged in regular transverse rows. 



The tentacles are compound in the sub-families Bolteninas and Cynthinse and resemble 

 those of the Molgulidse, but in the Styelinse they are simple and filiform as in the 

 Ascidiidse. 



Beitrage zur nahern Kenntniss tier Tunicaten, p. 15, Sitzber. d. k. Akad. d. Wiss., Bd. lxxvii., Abth. 1, 1878. 



