26 PHYLUM TUNIC AT A (UROCHORDA) 



grouping, so that it seems best to adopt a plan which has at least the merit 

 of being simple and easy of application. 



Tribe 1. ASCIDIAE SIMPLICES (MONASCIDIA). 



Solitary usually fixed forms, incapable of budding in the adult 

 state ; with large 'pharynx, and numerous branchial stigmata. 



These are the typical sea-squirts. They are solitary forms 

 usually of considerable size and attached by their tunics to rocks 

 and sea-weeds. A few however, e.g. Molgula, are not attached 

 save to grains of sand by processes of the test, and in a few 

 stalked forms the stalk arises from near the mouth (Boltenia. 

 Culeolus, Fig. 19). The tunic is usually somewhat thick and 

 opaque and may have a cartilaginous consistency. When touched 

 they frequently discharge two streams of water which proceed 

 from the two openings, the one. the mouth or inhalent opening 

 which is terminal and at the free end, the other the cloacal or 

 atrial aperture, which is placed on the dorsal surface a little 

 distance from the free end. The protective covering of mud or 

 sand is generally associated with fibrous processes of the test 

 (Molgulidae, Polycarpa molguloides. Ascidia conchilega), but in 

 some cases the sand adheres directly to the test. 



In Culeolus, Fungal us and Bath your us. abyssal forms belonging 

 to different sub-families of the Cynthiidae, the pharynx presents 

 the remarkable modification of being without the fine longitu- 

 dinal bars (Fig. 20). The dorsal lamina varies from the con- 

 dition of a membrane to that of a series of languets ; an inter- 

 mediate condition of a toothed membrane being found. 



The viscera are placed in the body- wall at the level of the 

 hinder part of the pharynx (except in Ciona), usually on the 

 left side. They project into the atrial cavity, sometimes being 

 attached to its wall by a mesentery. The atrial cavity is always 

 traversed by vascular strands passing from the pharynx wall to 

 the mantle \vall. The vascular system is well developed, the 

 sinuses sometimes having the aspect of vessels. There is usually 

 a tailed larva. 



Fain. I. Ascidiidae. Usually sessile, rarely pedunculate d : month 

 usually with S lohes, atrial aperture \\ it li <> : pharyngcal wall without folds, 

 \\itli internal Inngit u< linal hars hearing papillae; stigmata straight 01 

 enr\ ec I ; tentacles unl>raiiclu > d : gonads pi act d in t he intcst inal loop. ( 'iona 

 approximate^ t<> the ( 'la \ elinidae l>\ the position c t' its \ is< era ;iiid the 

 presence of an epieanlillin (pp. !.">. '.{'). 



