1881.] 



On the Genus Ciileolus. 



105 



is of great morphological interest. As it belongs to the Cynthiacl 

 type, it is necessarily so far complicated as to possess a certain number 

 of longitudinal folds on each side, but in all other respects it is the 

 simplest form of branchial sac known among Simple Ascidians. 



Neglecting for the moment the longitudinal folds, the organ may be 

 described as a simple network, formed by two series of vessels cross- 

 ing at right angles and communicating at the points of intersection. 

 The two series are the horizontal or transverse vessels, which are some- 

 times of two or more sizes occurring alternately, and the internal 

 longitudinal bars which run vertically and generally form the strongest 

 part of the network. This is the structure of the branchial sac 

 between two folds in the simplest form, Culeolus murrayi, and the 

 great difference between it and the simplest form of branchial sac in 

 the genus Ascidia (e.g., A. cylinclracea or A. venosa, where minute longi- 

 tudinal plication of the sac is not present) lies in the fact that in Culeolus 

 no fine longitudinal vessels are present, and consequently the meshes 

 are not broken up into stigmata. In two of the species, however, here 

 and there over the branchial sac, a mesh was found divided more or 

 less irregularly by a delicate longitudinal vessel crossing from one 

 transverse vessel to the adjacent one. These cases were rare, and evi- 

 dently abnormalities, but they indicate a tendency towards the division 

 of the mesh into stigmata through the development of fine longitu- 

 dinal vessels. In Culeolus perlucidus this process has taken place. 

 Here each mesh is divided into two equal areas by a delicate longitu- 

 dinal vessel running between the transverse vessels. In this species, con- 

 sequently, one might correctly describe the branchial sac as having two 

 stigmata in each mesh. Along the free edges of the internal longitudi- 

 nal bars the epithelium is cubical or low columnar, but never ciliated. 



One peculiarity of the branchial sac throughout the genus remains 

 to be mentioned. That is the presence in its vessels of an extensively 

 developed system of calcareous spicules. These are of considerable 

 size, often much ramified, and have a very charact eristic appearance 

 from their gentle curves and blunt ends. They vary in size, abund- 

 ance, and amount of branching according to the species ; and are chiefly 

 developed in the internal longitudinal bars, and along the edges of 

 the endostyle. 



The dorsal lamina throughout the genus is represented by a series 

 of triangular languets. 



The alimentary canal from the oesophageal opening onwards, 

 though differing somewhat in its details in the different species, has in 

 all the same general course. It lies on the left side of the branchial 

 sac, in its posterior half, and nearer to the ventral than the dorsal edge. 

 The oesophageal aperture (oe.a.) lies far back in the branchial sac, at 

 the posterior end of the dorsal lamina (d.l.) The oesophagus is short, 

 and runs ventrally to open into the large stomach (s.t.) which lies 



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