xra PHYLUM CHORDATA 21 



the larva. The adults, which for the most part are retrogressively 

 metamorphosed in other respects besides the abortion of the 

 notochord, are sometimes sessile, sometimes free and pelagic ; they 

 frequently form colonies (fixed or free) by a process of budding, 

 and in some instances exhibit a well-marked alternation of gene- 

 rations. The body is enclosed in a test consisting largely of 

 cellulose. The proximal part of the enteric canal (pharynx) is 

 enlarged to form a spacious sac- with perforated walls, acting as 

 an organ of respiration. There is a simple heart with a system of 

 sinuses, all devoid of epithelial lining. The ccelome is represented, 

 apparently, only by the pericardium and by spaces in the interior 

 of the gonads and of the renal organ. The sexes are united. The 

 larva is always free-swimming, and is nearly always provided with 

 a caudal appendage. 



Three orders of Urochorda are recognised : 



ORDER 1. LARVACEA. 



Free-swimming pelagic Tunicata with a caudal appendage 

 supported by a skeletal axis or notochord. The test is represented 

 by a relatively large temporary envelope, the " house," formed 

 with great rapidity as a secretion from the surface of the ectoderm 

 and frequently thrown off and renewed. The pharynx has only 

 two stigmata, and these lead directly to the exterior. There is no 

 atrial or peribranchial cavity. The principal nerve-ganglion gives 

 off a nerve-cord with ganglionic enlargements running to the tail, 

 along the dorsal aspect of which it passes to the extremity. There 

 is no reproduction by budding, and development takes place 

 without metamorphosis. 



This order contains only a single family, the Appendiculariidce, 

 with about nine genera, including Appendicularia and Oikopleura. 



ORDER 2. THALIACEA. 



Free-swimming Tunicata, sometimes simple, sometimes colonial, 

 never provided with a caudal appendage in the adult condition. 

 The test is a permanent structure. The muscular fibres of the 

 body-wall are arranged in complete or interrupted ring-like bands, 

 or diffusely. The pharynx has either two large or many small 

 stigmata leading into an atrial cavity which communicates with 

 the exterior by the atrial aperture. There is usually an alternation 

 of generations ; there may or may not be a tailed larval stage. 



Sub-Order a. Cyclomyaria. 



Thaliacea with a cask-shaped body, having the oral and atrial 

 apertures at opposite ends, and surrounded by a series of complete 

 rings of muscular fibres. There is a tailed larval stage. 



