TYPE PROTO CHORD ATA. 631 



bud may fail, these monsters may have either a double bran- 

 chial sac, atrium, and rectum, or a double digestive tract, heart, 

 and other organs of the visceral mass. If, however, both 

 develop simultaneously, in the course of time the perfect 

 daughter form separates from the parent. In Botryllus (Fig. 

 288) but a single bud (b) arises from each individual of the 

 colony, developing on one side of the body in the region of 

 the branchial sac, early separating from the parent except 

 for a slight stalk like union. After the formation of the 

 daughter individuals the parent forms die, and so a succes- 

 sion of generations occurs, the various individuals of each 

 generation being arranged in a circle around a common cloaca 

 (cl) into which the atrial aperture of each opens. 



The origin of a colony of Botryllus takes place as follows : An individ- 

 ual developing from an egg fastens itself and gives rise to a single bud, 

 which remains imbedded in the test of the parent, which, on its part, dies 

 and disappears. The individual of this second generation gives rise to two 

 individuals of the third generation and then likewise dies, while each of 

 the members of the third generation gives rise to two more members of a 

 fourth generation and degenerates. The four individuals so formed ar- 

 range themselves so that they radiate from a common point at which a 

 depression of the test occurs, forming the cloaca. New generations then 

 succeed each other, the parents of each generation in turn degenerating, 

 and so the colony extends, and some of the individuals failing to form a 

 connection with the original cloaca become centres for a new radiating 

 colony, still, however, imbedded in the common test. 



3. Suborder Pyrosomidm. 



In this order there is but a single genus, Pyrosoma, which 

 includes floating pelagic colonies having the shape of a cylin- 

 der closed at one end, the individuals composing the colonies 

 being enclosed in a common test and arranged radially around 

 the central cavity or cloaca into which their atrial apertures 

 open, the branchial apertures opening on the exterior of the 

 cylinder. Each individual resembles in structure a simple 

 Ascidian, the principal difference being that the atrial aper- 

 ture, as in Botryllus, is at the posterior end of the body, and 

 that each individual has the power of reproducing by budding 

 and so assisting in the further growth of the colony, the 

 parent forms not, however, degenerating after giving rise to 



