UROCHORDA. 441 



SUB-PHYLUM AND CLASS UROCHORDA (=Tunicata). 



Degenerate Chordata, which are either simple or compound (colonial), 

 fixed or free ', with a test usually containing cellulose ', and secreted by the ecto- 

 derm. The nervous system is reduced to a single ganglion, except in one group 

 (Larvacea) ; the notochord confined to the tail, which, with the exception of the 

 same group, is a provisional or larval structure. There is a single or double 

 exhalent aperture by which the water escapes that has traversed the pharynx 

 Hermaphrodite. There is usually a metamorphosis. 



The class contains three distinct orders. The Larvacea are small in 

 size, and have a flattened swimming tail moveably attached near the 

 posterior extremity of the body on its ventral surface, towards which it 

 bends. The Thaliacea are free-swimming, and more or less barrel-shaped. 

 Among the Ascidiacea, the Ascidiae Salpaeformes (Pyrosomd) are colonial, 

 and the colony has the form of a cylinder closed at one end, open at the 

 other, with the Ascidiozooids placed vertically to the surface, in a common 

 test or Ascidiarium ; the A. Compositae form fixed colonies with the 

 Ascidiozooids contained within an Ascidiarium, sometimes grouped into 

 systems or coenobii ; and the A. Simplices are fixed, and either solitary 

 and of a compressed shape, or united by creeping stolons (Clavelinidae\ 



The ectoderm consists of a single layer of polygonal cells. The cells 

 of the anterior region of the body in Larvacea are large, and secrete a 

 hyaline gelatinous and sticky substance, which forms the so-called ' house.' 

 This house is sometimes of great size, and extends over the swimming tail. 

 It is separated by one cavity from the greater part of the body, and by 

 another from the tail, which moves freely within. It is detached and 

 renewed at short intervals. When the house is small, the ectoderm cells 

 left uncovered by it are modified, and in Fritillaria urticans some of them 

 develope thread cells. The test is very thin and delicate, and is shed from 

 time to time in Doliolum. It is thicker in other Urochorda, and either 

 transparent or when very thick more or less opaque, gelatinous or tough, 

 and in the Ascidiae Compositae and Salpaeformes it constitutes a con- 

 tinuous investing mass in which the zooids are lodged. The substance of 

 the test is chiefly cellulose except in Doliolum ; it is often fibrillated, and 

 in some A. Compositae calcareous deposits occur in it, especially in autumn. 

 The cells of the ectoderm generally proliferate and wander into it, and the 

 cells thus imbedded may become pigmented or vacuolate. It is also often 

 penetrated by blood-vessels. The body walls are composed of muscular 

 and connective tissues. The muscle fibres are fusiform or filiform, and not 

 striated. The fibres form sphincters round the oral and the atrial apertures. 

 Elsewhere they may be arranged either irregularly, or in longitudinal and 

 circular layers, or in circular hoops round the barrel-shaped body (Thaliacea) 

 which are either independent (Cyclomyaria) or united (Desmomyarid). In 



