II. TUNICATA: COPELAT.E 443 



ceived by a ring-shaped ciliated band (peripharyngeal band) and, envel- 

 oped by mucus, are led to the oesophagus. This mucus is formed by 

 a ciliated glandular groove, the endostyle (e) or hypobranchial groove, 

 on the ventral surface of the pharynx. 



Between the end of the endostyle and the stomach lies the ventral 

 heart enclosed in a pericardium. It has the peculiarity, met nowhere else, 

 of changing the direction of its contractions at frequent intervals ; after the 

 heart has driven the blood for a time to the gills it stops and then forces 

 the blood in the opposite direction, pumping it from the gills towards the 

 stomach. If we add to the foregoing that a dorsal ganglion and a usually 

 hermaphroditic gonad are present, the striking features of the group are 

 enumerated. The extreme forms, the Copelate and the Thaliacea, are 

 rather remote, but they are connected by intermediate forms, the Ascidke 

 and Pyrosomas. 



Order I. Copelatge. 



These small forms one or a few centimeters in length are pelagic; they 

 have the anterior end inserted in a gelatinous envelope or 'house' which replaces 

 the lacking tunic and which they may leave without injury. They swim like a 

 tadpole by means of a tail which arises from the hinder end of the trunk. The 

 alimentary canal (fig. 497) is bent on itself, and both it and the two large gill 

 slits, in contrast to all other tunicates, open directly to the exterior. The nervous 

 system consists of a cerebral ganglion, with beside it a very simple statocyst and 

 a ciliated groove, and farther a chain of ganglia extending into the tail. The 

 gelatinous notochord, enclosed by a sheath of cells, forms the skeletal axis of the 

 tail ventral to the nerve cord and gives attachment to muscles. Oikopleura,* 

 Appendicularia* 



Order II. Tethyoidea (Ascidiaeformes). 



With the exception of the pelagic Pyrosomidae all of the ascidians are 

 attached to rocks, etc., in the sea. The necessity for protection caused 

 by this sedentary life has resulted in a great development of the cellulose 

 tunic or test, which gives these animals a swollen, somewhat shapeless 

 appearance. Two openings, mouth and atriai opening, lead into the 

 interior, and the water which issues from these, when the animals are 

 taken from the ocean, has given them the common name of 'sea-squirts.' 



On removing the tunic, which is but slightly attached to the other 

 parts except at mouth and atriai opening, a muscular sac is seen (fig. 498), 

 the fibres running circularly and longitudinally. Inside this sac are the 

 viscera, the pharyngeal region by far the most conspicuous. The mouth 

 leads to a short tube with tentacles (/), and then to the pharynx, a wide 

 sac which lies in a large peribranchial chamber, the walls of the pharynx 

 and the enclosing space uniting on the ventral side (fig. 496). The 

 pharyngeal walls are perforated like a net by small ciliated gill slits, 



