410 



UROCHORDA 



[CH. 



reversed, so that the blood alternately goes to the dorsal vessel from 

 the heart and vice versa. With the exception of the heart, however, 

 the blood-vessels do not seem to have definite walls, and must be 

 regarded like those of the Hemichorda as crevices left between 

 various organs. 



The sluggish life of the Tunicate has as its only external 

 manifestation the sudden closing of the mouth and atrial cavity 

 by sphincters, and the consequent ejection of water whence the 

 popular name Sea Squirt. In consequence metabolism is at a low 

 level and not much waste is produced. A good deal of this 

 waste is probably got rid of by the throwing off of the mantle from 

 time to time, but for the rest no definite excretory organ is required. 

 The nitrogenous excretion is stored up as crystals of insoluble uric 

 acid in little vesicles attached to the hinder part of the intestine. 

 These vesicles, together with the cavities of the genital organs and 

 the pericardium, may be looked on as the remnants of the coelom, so 

 that here a similar phenomenon has taken place to what was met 

 with in the case of Arthropods, namely an obliteration of the coelom 

 through the expansion of blood-vessels. 



The Tunicata or Urochorda abound on every rocky shore and 



exhibit a surprising diversity of 

 form. Their principal divisions 

 are as follows. 



Class 1 COPELATA or LARVACBA. 



Small forms which retain the 

 larval condition throughout life. 

 The gill-slits are undivided and 

 the anus is ventral. There is no 

 atrial cavity : each of the two 

 gill-slits opens directly to the ex- 

 terior. The tail is usually 

 carried bent forward at a sharp 

 angle with the body. A tem- 



Fio. 204. Two groups of individuals porary test devoid of blood- 

 of Botryllus vioiaceus, after Milne vessels is found : the animal 

 Edwards. Magnified. , ,. , , , . , 



when disturbed wriggles out of 



1. Mouth opening. 2. Common ., , ~ ., 



cloaca of the group. it and forms another. 



