476 TUNICATES 



We have spoken so far of the mouth and the respiratory 

 pharynx, the rest of the food-canal — the digestive and absorptive 

 region — lies like a distorted S on the left-hand side. It is pos- 

 sible to distinguish a gullet, a stomach, and an intestine ; and 

 there is often a definite gland opening into the stomach. Along 

 the whole length of the intestine there is an inturned pad (the 

 typhlosole) which helps to increase the digestive absorptive 

 surface. A similar arrangement is found in the earthworm. 

 Covering the walls of the intestine to a greater or less extent 

 are curious clear vesicles without openings which contain uric 

 acid and are without doubt of the nature of kidneys. The 

 nitrogenous waste-products are filtered out of the blood, but 

 they are not directly eliminated from the body. This is a fact 

 of peculiar interest ; it suggests a constitutional defect that may 

 have something to do with the sluggishness. We may recall 

 the fact that plants do not get rid of their nitrogenous waste- 

 products, but accumulate them within the body (sometimes in 

 the form of crystals or of pigment). We may note also the 

 retention of nitrogenous waste in many winter-sleepers, and the 

 fact that coma sometimes supervenes in man when the kidneys 

 cease to filter out the poisonous nitrogenous waste. It may be 

 that the Tunicates suffer from auto-intoxication with uric acid. 

 To some extent, however, helped by bacteria in the vesicles, the 

 waste may diffuse out. 



We have seen that all that remains of the larval spinal cord 

 and brain is a single elongated ganglion between the two 

 apertures. It gives off nerves at both ends to the lobes of the 

 apertures and the muscles which close them. On its ventral 

 surface there is a small "sub-neural gland " of unknown function, 

 which has a duct running forwards and opening into the front 

 of the pharynx at a sensory projection called the dorsal tubercle. 

 It is maintained by some authorities that the sub-neural gland 

 corresponds to the pituitary body of higher animals (a minute 

 downgrowth from the floor of the brain meeting a minute 

 upgrowth from the roof of the mouth). The adult sea-squirt 

 has neither eye nor " ear," though its larva has both. There is, 

 however, considerable sensory development about the beginning 

 of the pharynx, e.g., in the circlet of delicate inturned tentacles 



It is one of the contrasts between Invertebrates and Verte- 

 brates that the heart lies dorsally in the former and ventrally 



