APPENDIC ULARIAE. 



61 



The Perennichordata are free-swimming pelagic organisms 

 provided with a locomotory tail much longer than the body. 

 They usually occupy a cavity, much larger than themselves and 

 opening to the exterior at one or more points, in a gelatinous 

 capsule which corresponds to the test of other Tunicata (Fig. 54), 

 They are capable of moving freely in the capsule by the undula- 

 tions of the tail, and of creating currents of water which flow 

 in and out of the capsule to 

 supply them with food and 

 oxygen. The test or capsule 

 is readily cast off, and the 

 animal then swims freely in 

 the sea, with the hind end of 

 the body forwards. But a 

 new capsule is rapidly formed 

 as a cuticular secretion, not 

 of the whole ectoderm, but 

 of certain parts of it (the 

 so-called oikoplasts). The 

 test contains a few cells, but 

 no cellulose has been detected 

 in it. 



The tail, which contains 

 an axial notochord and has 

 two lateral cutaneous ex- 

 pansions, is attached to the 

 ventral side of the body at 

 some distance from its hind 

 end (Fig. 55) The mouth 

 is at the front end, and the 

 anus on the ventral surface 

 anterior to the insertion of 

 the tail, either in the middle 



line or a little to the right of the middle line. The gill-apertures, 

 often called spiracles, are placed laterally either in front of or 

 behind the anus. The genital organs are in the hinder part of 

 the body behind the insertion of the tail. 



The body- wall consists of a simple layer of ectoderm, thickened 

 and glandular in places. There is no dermis, and muscles are 

 usually absent in the region of the body, except in Megalocercus. 



FIG. 54. Oikopleura cophocerca in its test (after 

 Fol, from Seeliger). There are two superior 

 openings, but one only is shown. As the 

 arrows indicate, water passes in by them and 

 out by the lower opening. 



