TtKrORT ON THE TUN1CATA. 11 



Tentacles (Branchial Filaments). 



The tentacles have been already referred to, and (heir position at the posterior end 

 of the branchial siphon described (fig. 3, tn., p. 32). They are hollow processes, not 

 of the involuted test but of the wall of the alimentary canal, and of the mantle, as these 

 two are in contact in that region, the peribranchial space not extending so far forward. 

 The epithelium covering the tentacles is continuous with that of the praabranchial zone, 

 while the connective tissue underneath, usually containing muscle bands, is in connection 

 with that of the mantle. Blood sinuses from the latter are also prolonged up the centri 

 of the tentacles, so that the blood circulates in their interior. 



They vary greatly in size and shape in different genera and species, and very 

 frecpuently they are of different sizes, arranged symmetrically. In a few cases (<■</., 

 Culeolus wyville-thomsoni and Molgula pedunculate) one of the tentacles is very much 

 larger than any of the others. 



In the Molgulidae the tentacles are always compound, and are usually much branched, 

 in some of the larger species (e.g., Molgula gigantea) forming huge complicated 

 arborescent masses. Their number in the Molgulidaa is never very great. A not 

 uncommon arrangement is for seven or eight large tentacles to alternate with the same 

 number of smaller ones. 



In the Cynthiidae we find a great variety of tentacles. Two of the sub-families, the 

 Bolteninae and the Cynthinae, have compound tentacles very similar to those of the 

 Molgulidae, but usually more numerous, while the third sub-family, the Styelinae, has 

 simple unbranched tentacles like those of the Ascidiidae, but frequently wider at the base, 

 and having a more inflated appearance. 



In both the Ascidiidae and the Clavelinidae they are also simple, and present the form 

 of long tapering processes, usually distinctly triangular in cross section, and placed with 

 a flat surface anteriorly, and a ridge posteriorly. Among the Ascidiidae we frequently meet 

 with complex arrangements. The tentacles may be of different sizes, two, three, or 

 more orders being placed alternately. 



In the Ascidiae Compositae the tentacles are invariably simple, and few in number. 

 They are usually all of the same length, although in some cases (e.g., Botryllus, where 

 there are four larger alternating with four smaller) two orders occur. 



In the Appendiculariidae, the Pyrosomidte, and the Thaliacea the tentacles seem to be 

 entirely absent. 



Nervous System. 



A single large nerve ganglion is found in all Tunicata placed between the branchial 



and atrial siphons, and is usually considered as indicating the dorsal surface of the 



