200 



THE VOYAGE OF H.M.S. CHALLENGER. 



d&pressa, Alder), a series of smaller intermediate and alternating papillae is present on 

 the internal longitudinal bars in addition to those at the angles of the meshes. 



In the simplest condition the transverse vessels are all of one size, and the interstig- 

 matic vessels connecting them are placed in one plane, forming a continuous row (fig. 20) ; 

 but in many species (e.g., Ascidia meridionalis) there are two or more distinct sizes 

 of transverse vessels, which are usually placed so as to alternate regularly ; and in 

 some species the fine longitudinal or interstigmatic vessels are arranged so that their 

 line of insertion into the transverse vessels forms an undulating in place of a straight line 

 (fig. 21), or is even more irregular (PI. XXIX. fig. 4). 



(rough 



tr 



11 



..rd. 



Fig. 21. — Diagrammatic horizontal section through a Mesh of the Branchial Sac of Ascidia, showing the arrangement of the fine 



longitudinal vessels which causes "minute plication." 



tr. ..transverse vessel ; l.v., fine longitudinal vessel ; i.L, internal longitudinal bar ; c.d., connecting duct ; p., papilla ; h.m., horizontal 

 membrane. " Crest " indicates the highest, and " trough " the lowest part of the undulation. 



The result of this arrangement is that the sac, when seen from the inside, or still 

 better from the outside (PI. XXXI. fig. 5. and PI. XXXIII. fig. 3), where there are no 

 internal longitudinal bars to obscure the view, seems to be thrown into a series of 

 minute crimps or plaits, and this is the structure which has been called " minute 

 plication." 



Verrill, 1 in 1872, suggested that Ascidia complanata, Fabricius, should form the type 

 of a new genus, to which he gave the name Ascidiopsis, on account of the remarkable 

 structure of its branchial sac ; but from the figure given, this structure seems to be merely 

 the above described minute plication which is found in Ascidia mentuia, Ascidia 

 virginea, and a number of the oldest known and most typical species of the genus 

 Ascidia. The plication varies considerably in the degree to which it is present, and also 

 in its regularity. Usually when the sac is plicated there are several sizes of transverse 

 vessels, and the smaller sizes may enter into the undulations of the stigmatic vessels, 

 while the larger ones do not. In this case the appearance of the outer surface of the sac, 

 when the plication is regular, is that of a number of alternately placed projections and 



1 Descriptions of some imperfectly known and new Ascidians from New England, Aiuer. Journ. Sc. and Arts, ser. 

 ill- , vol. iii., No. 16. 



