SYNASCIDIA. 



31 



the pharynx ; test usually soft, with numerous vessels ending in terminal 

 knobs and joined to the body of each zooid at two points ; pharynx large 

 with 3 internal longitudinal bars on each side and numerous stigmata ; 

 dorsal lamina as a membrane ; tentacles simple, not more than 16 ; gonads 

 on both sides in the mantle (except Symplegma) ; gemmation lateral, from 

 the bodies of the zooids* ; the neural gland is dorsal to the ganglion in 

 Botryllus ; the stomach has an hepatic caecum. 



The budding of Botryllus differs from that of other synascidians in that 

 the endoderm does not participate ; the bud being formed as an outgrowth 

 of the atrial cavity and consisting only of outer ectoderm of the body, 

 ectodermal lining of the atrial cavity and interposed mesoderm (Fig. 24). 

 The process begins in the larva before hatching, as a pair of ventral out- 

 growths of the atrial cavity. After fixation of the larva the left of these 

 atrophies and the right alone de- 

 velops. The zooid produced from it 

 gives rise to two buds by a process 

 which is described below and is re- 

 peated in all the subsequent budding. 

 The zooids of the colony thus in- 

 crease in geometrical ratio, but in 

 all cases when the buds are developed, 

 the form which has produced them 

 dies ; thus the fixed larva dies when 

 it has produced its bud, and the 

 latter dies when it has developed and 

 produced its two buds. The zooids 

 produced arrange themselves so that 

 their atrial cavities are turned to- 

 wards one another and open into the 

 common cloaca which is a depression 

 of the surface of the colony as in 

 Pyrosoma. A system of zooids thus 

 arises. The number of zooids in a 

 system is limited ; when the limit is 

 reached, of the two buds which each 

 zooid produces one atrophies, and 



the other, instead of taking up its FlG - 23 - Botryllus violaceus (after M. Ed- 

 wards from Claus). mouth; A opening 

 position in the system, moves away of common cloaca of a system. 



from it and becomes the centre of a 



new system in the same colony. The budded zooids do not separate from 

 the parent as in most synascidian colonies. It is true that the pharynx of 

 the bud loses its connection with the atrium of the parent, but the outer 

 ectodermal connection persists and becomes an elongated and slender tube 

 by which the vascular systems of the parent and bud remain in continuity. 

 On the atrophy of the parent zooid the two tubes which connected her 

 to her offspring become directly continuous owing to the fact that though 

 the internal organs of the parent break down, its ectoderm persists. 



As stated above the buds are formed as diverticula of the atrial cavity 

 on its ventral side (Fig. 24). They form hollow vesicles, the cavity of 

 each of which divides into two ; one of these becomes the pharynx and the 



* In Sarcobotrylloides Herdman describes stolonial bxidding from the 

 vessels of the test. 



