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



become spirally coiled, and both, these changes have been caused by modifications of the 

 fine interstigmatic vessels. The most abnormal form of branchial sac is that found in 

 the genus Hypobythius, where there are no internal longitudinal bars. I am inclined to 

 believe this to be a secondary modification, since we occasionally find in species of 

 Ascidia, and especially of Corella (PI. XXVI. fig. 8), places where the internal longi- 

 tudinal bars are irregular and partially absent. 



The dorsal lamina is found in several different conditions in the Ascidiidse. Probably 

 the primitive state was a series of languets, which were outgrowths from the dorsal line 

 (A' the sac, corresponding to the connecting ducts arising from the transverse vessels. 

 This simple condition is found in Ciona, and several other genera, while in Ascidia and 

 Pachychlama the languets have become more or less completely united by a membrane, 

 the dorsal lamina proper, but still show traces of their original arrangement by projecting 

 in the form of transverse ribs and marginal teeth. The simpler condition of a series of 

 tapering languets is found constantly amongst the lowest family of Simple Ascidians, the 

 Qavelinidae, whde the more modified and probably much more efficient organ, the plain 

 lamina, prevails in the highest and most complex family, the Molgulidse. In the inter- 

 mediate families, the Ascidiidse and the Cynthiidse, both conditions and all the transition 

 stages by which the one graduates into the other are found. 



Neither of the two previously known genera of the Ascidiidse in which the test is 

 remarkably modified, namely, Rhodosoma and Chelyosoma, were collected during the 

 Challenger expedition, but two of the new forms show notable peculiarities in the test, 

 Packychiama having it greatly thickened all over, while Hypobythius ealycodes, Moseley, 

 has a series of symmetrically placed nodular, cartilaginous thickenings in the otherwise 

 thiu and membranous test. 



A few lines will suffice for the faindy Qavelinidae, as I shall leave the discussion of re- 

 production by gemmation for the second part of this Report ; and the affinities of the new 

 Challenger genus Ecteinascidia have already been fully discussed in the systematic part. 

 In this genus, and still more in Clavelina, all the organs are found in a simple condition, 

 not unlike that seen in Ciona among the Ascidiidse. In Clavelina internal longitudinal 

 liars are totally wanting, the stomach and intestine extend beyond the branchial sac, and 

 the dorsal lamina is in the form of lan°-uets. 



Two of the species of Ecteinascidia (Ecteinascidia fusca and Ecteinascidia turbinata) 

 supply us with evidence as to the homology of the languets with the connecting ducts 

 of the branchial sac. In Ecteinascidia fusca the connecting ducts are broad and 

 triangular (PL XXXVI. fig. 8, c.d.), and the languets (PI. XXXVI. fig. 11, I.) are also 

 flattened and triangular, although of course longer, while a short distance at each side of 

 them, just at the right and left edge of the dorsal area of the sac, is a row of processes very 

 simdar in every way to the connecting ducts, but having no internal longitudinal bars 

 attached to their apices, which hang freely, like those of the languets (PI. XXXVI. fig. 11). 



