ORGANS OF DIGESTION. 363 



visible the course of the convoluted digestive canal through 

 the abdominal cavity, where it is generally found ascending 

 on the right side of the respiratory or thoracic cavity in the 

 higher isolated forms of tunicata, and is contained beneath 

 that cavity in many of the lower compound forms. As the 

 short component follicles of the liver open freely into the 

 alimentary canal, their contents are often tinged with the 

 colour of the food, and thus give rise to a diversity of colour 

 in the liver of these animals. In the cynthia dione, the 

 hepatic follicles are long and isolated, and envelop the 

 stomach, as the pancreatic follicles or caeca pylorica envelop 

 the pylorus in most osseous fishes, and in this cynthia, as in 

 some others, the turns of the intestine are in contact with 

 each other throughout their course, and the buccal and anal 

 orifices are nearly approximated. In the botryllus and other 

 compound tunicata, each component animal has its own 

 distinct organs for nutrition and generation, constructed on 

 the same plan as in the isolated species, the respiratory 

 orifices of the botryllus open separately on the surface, around 

 a large central aperture which gives exit to all the ex-currents 

 of the separate vents. The prominent papillae which cover 

 the exterior surface of the pyrosoma (Fig. 119. 2 c.) have 

 each a respiratory orifice near their apex, and the currents 

 pass through the body of each component animal to the anal 

 apertures situate in the interior of the general tube formed by 

 the aggregation of all the individuals. Near the bottom of 

 each reticulated ciliated respiratory sac is the small round 

 buccal orifice leading by a short narrow O3sophagus to a 

 simple globular stomach ; the intestine, furnished with a 

 distinct liver, forms a single convolution, and terminates near 

 the vent. On opening the general tube of the pyrosoma the 

 numerous small vents of the respiratory sacs are observed, by 

 which the currents pass into the tube and move it through 

 the sea. 



XV. Conchifera. The general plan of structure in the 

 digestive apparatus of the inhabitants of bivalve shells is 

 very similar to that of the tunicated mollusca, but they 

 present a more complicated and higher condition of de- 

 velopment in the several organs. The conchiferous ani- 

 mals are commonlv fixed or slow in their movements, and, 



