ALIMENTARY CANAL OF TUNICATA. 403 



remarkable modifications. The cells form up-standing* longitudinal 

 ridges. At the bottom of the groove, between the two most deeply 

 embedded ridges, there are cells provided with long vibratile filaments 

 which may even project into the respiratory cavity. The Copelata 

 possesses the simplest arrangement of this organ. In many there is 

 only a single cellular ridge. Two are recorded in Doliolum. Three 

 occur together, with other complications, in the AscidiaB and Salpas. 

 The margins of the groove as a rule lie in close apposition, so that 

 the groove is closed except at one part near its foremost extremity. 

 At this spot commence the ciliate tracts which embrace the 

 entrance to the respiratory chamber. They are clefts beset with cilia- 

 bearing cells which take a dorsal course and lead either to the 

 oesophagus (Copelata) or to the neighbourhood of the great ganglion, 

 taking a spiral turn on the way (Doliolum), or end in a ciliated 

 groove (Salpas). A similar division of the foremost section of the 

 intestinal tract was noted in the Enteropneusti (Fig. 211, A). Two 

 longitudinal folds (*) separate this region into a respiratory (r) and 

 an alimentary (n) portion. The latter seems to be strictly comparable 

 with the ventral groove of the Tunicata, which in its early stage of 

 development is also of very considerable dimensions relatively. 



The function of the ventral groove is that of a glan- 

 dular organ. The cellular ridges secrete a slimy substance which 

 is carried forwards to the open part of the ventral groove by the 

 cilia placed along its floor, and from thence is further moved along 

 the ciliate tracts. At the same time the mucous masses project into 

 the lumen of the respiratory chamber, arrest the nutrient particles 

 taken in with the water, and become formed with these particles 

 into a string which passes into the oesophagus. Since too the free 

 margins of the ventral groove are beset with cilia, and a ciliate tract 

 can be traced as far as the oesophagus, the mucus also which passes 

 out of the slit of the groove is caught up by the cilia, and directed, 

 together with the nutrient particles sticking to it, towards the 

 oesophagus. Briefly, the ventral groove secretes mucus, which is 

 destined to catch the nutrient matters suspended in the water, and 

 to be carried with them by the ciliate tracts to the oesophagus. The 

 entire arrangement has therefore a nutrient signification. 



Fol, H., Ueber die Schleimdriise, etc. der Tunicaten. Morph. Jalirb. I. p. 223, 



Digestive portion of the Enteron. 



§ 312. 



At the bottom of the foremost section of the entire enteron or 

 tractus intestinalis, namely, of that section which is modified as a 

 respiratory chamber, commences the tract which serves exclusively the 

 functions of nutrition. As a rule several divisions of it may be dis- 

 tinguished by differences in its breadth. An anterior, narrow section 



2 d 2 



