ASCIDIAN. 



atrial aperture. The system of internal longitudinal vessels is very well developed 

 in Ascidiidae, and it is characteristic of the family to have a prominent papilla 

 developed from the free surface of these vessels at spots opposite to the short 

 vessels which connect them to the transverse system. 



The endostyle is a groove, at the bottom of which are cells bearing extremely 

 long cilia. The sides of the groove are ridged, and the ridges are caused by 

 the large size of certain cells which appear to secrete mucus. The mucus col- 

 lects into small lumps or balls which are carried forwards to the peripharyngeal 

 groove. They traverse this groove and are then conveyed by the ciliated dorsal 

 lamina backwards to the stomachal opening. The dorsal lamina has anteriorly 

 a median epipharyngeal groove, the extent of which varies in different Ascidians. 

 The free edge of the lamina is curved, usually to the right. It ends posteriorly 

 in a low ridge continuous with the endostyle. 



The ganglion is single, generally swollen at either end where it gives off 

 an anterior and posterior set of nerves. The central part is fibrous, and the 

 ganglion cells are placed peripherally. The ganglion lies in the body walls, and 

 always between the oral and the atrial apertures, on the opposite side of the body 

 to the endostyle. As to organs of special sense, the tentacles must be regarded 

 as tactile ; and in many Ascidians there are orange-coloured visual (?) spots between 

 the lobes of the oral aperture. The sub-neural gland consists of caecal ramified 

 tubes. They open into a duct which lies between the gland and the ganglion. 

 This duct runs forwards and opens into a ciliated depression of the prae-branchial 

 zone. The margins of the depression are prominent, and the aperture has usually 

 a crescentic shape, the concavity of the crescent being turned forwards. The 

 margins may be much modified in shape. They constitute what is often spoken 

 of as the ciliated sac or dorsal tubercle. The gland has been homologised by 

 Julin with the pituitary body of the brain in higher Vertebrata, and it has been 

 suggested that it has a renal function. In some cases it has secondary ducts 

 opening into the atrial cavity. 



The digestive portion of the alimentary canal is disposed variously in different 

 groups of Urochorda, The liver (?) may be represented by glandular tissue 

 coating the stomach, and sometimes attaining considerable size, or by a system 

 of clear tubes ramifying over the stomach and part of the intestine and opening into 

 the pyloric portion of the former. A renal organ is probably represented by clear 

 walled vesicles containing concretions in which uric acid has been found, and 

 situated round the intestine and in the body walls. These vesicles have no ducts. 



The heart is more or less fusiform, and is inclosed in a delicate pericardium. 

 It lies in Asridia along the posterior ventral edge of the stomach on the left side. 

 It gives off (i) a ventral vessel which sends a branch through the mantle to the 

 test and then runs forwards beneath the endostyle, and is connected to the 

 transverse system of vessels in the walls of the pharynx; (2) a dorsal vessel 

 from its opposite end which sends a branch through the mantle to the test 

 parallel to the one above mentioned. These two branches divide in the test, 

 end in ampullae, and intercommunicate. The remaining branches of the dorsal 

 vessel go to the body walls; and to the stomach intestines and reproductive 

 organs from which the blood is collected into a large vessel running along the 

 dorsal edge of the pharynx and connected to its transverse vessels. There are 



