Dr. A. Krohn on the genus Doliolum and its species. 121 



presents the most striking deviation from the ordinary arrange- 

 ment. Instead of a sac, it forms a partition stretched across the 

 cavity of the body, flat in one species, bent at an angle in 

 another ; and dividing the space into an anterior and a posterior 

 compartment. 



Its structure is much simpler than that of the compound 

 Ascidians, since it is pierced by only two series of symmetrical, 

 transverse, or somewhat oblique clefts {" stiff mates branchiaux" 

 Milne-Edwards), the edges of which are, as in all Ascidians, beset 

 with ciha. In the median portion of the branchial membrane 

 the clefts are wider, beyond it they narrow again. - "^^ <avio\ 

 ,^ These clefts then are the only means of communicatibii df tne 

 anterior and posterior divisions of the cavity of the body with one 

 another. 



Upon the walls of the anterior division, the ciliary apparatus 

 for conducting food to the mouth is arranged ; i. e. the well-known 

 ventral groove and its prolongations, which are less known, and 

 may be here more minutely described. The anterior end of the 

 relatively short ventral groove gives ofi" two narrow ciliary bands, 

 which diverge from one another and run up at the base of the 

 respiratory siphon, along the parietes of the cavity of the body 

 to the dorsal surface ; here, converging towards one another, they 

 become united in front of the nervous ganglion. A third ciliated 

 band mns from the posterior extremity of the ventral groove tp 

 the mouth *. '[ Y.i isiii J «i)3 ^ )inof)3a 



Upon the wall of the posterior space lie the i'e|)rbdUfetiV3 or- 

 gans and the alimentary canal ; and anteriorly to the latter, in 

 its pericardial cavity, the heart, which pulsates very rapidly, and 

 is, as in the Salpce, a short sac. The circulation and the course 

 of the blood are not to be perceived, as the blood is pellucid and 

 contains no granules. 



The alimentary canal is but moderately developed in propdir- 

 tion to the size of the body. The mouth is placed upon the 

 branchial membrane, upon the great longitudinal ridge between 

 the lateral clefts. It leads into a short oesophagus, to which the 

 rounded stomach with the intestines bent into a loop succeeds. 



Like the compound Ascidians and the genus Clavelina among 

 the simple forms, Doliolum propagates both by ova and by buds. 



* A similarly constructed ciliary apparatus is to be found in all Tunicata, 

 according to my investigations. The anterior cihated band, forming a com- 

 plete circle, has been described by some zoologists as a vascular ring, some- 

 times as a nervous ring. So also an accessory part of the same apparatus, 

 especially frequent in the Cynthia as a rounded prominence, has been re- 

 garded sometimes as a nervous centre, sometimes as an organ of peculiar 

 structure and doubtful function (see Siebold, Vergleichende Anatomic, 

 p. 260). This elevation is distinguished, however, by no other circumstance, 

 than by being marked upon its surface by a spiral ciliated groove. '*'^'' 



