12S Mr. T. H. Huxley on the genus Doliolum. 



sure, however, that in the specimens I examined the relations of 

 the organ were as I have described and figured them. 



From the excellent description of the development of Doliolum 

 given in Dr. Krohn's memoir, it seems highly probable that my 

 guess as to the nature of the " shrivelled tubular process,^' p. 601, 

 is correct, viz. that it is the remains of a pedicle of attachment. 

 In common with all previous observers, Dr. Krohn appears 

 to have confounded what I have called the '' endostyle " with 

 the true " dorsal folds ^^ of Savigny. Recent careful examina- 

 tions of many species of Ascidians have convinced me that the 

 distinction which I drew between these structures (on Salpa and 

 Pyrosoma, 'Phil. Trans.' 1851, p. 572) is well founded. The 

 " endostyle '^ invariably exists at the base of the " dorsal folds " 

 in ordinary Ascidians, and consists essentially of a band of thick, 

 cylindrical, elongated cells, arranged round a common axis. Two 

 similar accessory bands are in the ordinary Ascidians developed 

 upon the folds on each side of the " endostyle.^' 



Dr. Krohn does not seem to have noticed the ciliated sac, or 

 the peculiar manner in which the anterior ciliary bands termi- 

 nate at this part. I have described similar bands in Salpa and 

 Pyrosoma {loc. cit, § 17-52), and I find that such exist in all 

 Ascidians. The " accessory part of the same apparatus " men- 

 tioned by Dr. Krohn is the " tubercule anterieure '' of Savigny. 

 It is not, as Dr. Krohn supposes, a mere appendage of the ciliated 

 bands, but it is a very peculiar structure placed in the space be- 

 tween the ciliated bands and the tentacular circlet (in ordinary 

 Ascidians), and is always in more or less close connexion with 

 the ganglion. It is the same organ as the "ciliated sac" of 

 Salpa, Pyrosoma, and Doliolum, and is, I think, very probably 

 an organ of sense. I have found it varying very remarkably in 

 shape and size in species of Boltenia, Cynthia, Molgula, and 

 Phallusia. 



The existence of a well-developed testis in Appendicular ia 

 (Vexillaria) [loc. cit. § 84) appears to me to present an insu- 

 perable difficulty to Dr. Krohn's hypothesis, that this creature is 

 an incompletely developed Ascidian ; and in addition to this cir- 

 cumstance, there is the absence of a cloaca (the anus opening 

 directly on the dorsal surface (§ 82)), which stamps the form as 

 altogether peculiar. 



With regard to the muscular apparatus of the tail of Ascidian 

 larvae, I may here state as a fact, which I believe to be alto- 

 gether new, that it is composed of a layer of large, elongated, 

 thick walled cells applied end to end. The cells contain a large 

 clear nucleus with a nucleolus. Their walls present a delicate 

 fibrillation^ which is continued from one cell to another, so that 

 it appears at first as if the cells were inclosed within a bundle of 



