TETHYODEA. 99 



soon become visible. The blood with its amoeboid corpuscles is 

 already moving in the body cavity beneath the skin, and indeed on 

 the branchial sac, through definite channels in the connective tissue, 

 which connects the walls of the branchial sac with the skin. The 

 water which flows through the slits of the branchial sac is collected 

 in the peribranchial space the opening of which coincides with the 

 cloacal opening. 



Asexual Reproduction. In addition to the sexual reproduction, 

 multiplication by means of budding plays an important part, particu- 

 larly in the Synascidians. According to Krohn, Metschnikoff, and 

 Kowalevski, an entodermal layer (arising in Botryllus from the cover- 

 ing of the atrium) and mesodermal cells as well as the ectoderm take 

 part in the formation of the buds. Many Ascidians, as Perophora 

 and Clavellina, produce stolons by budding, and from these new 

 individuals are developed, but the latter are not united together 

 into a compact system. Complex systems of buds are developed in 

 the Synascidians, the individuals of which are embedded in a common 

 cellulose mantle. In some cases the larva may form buds while it is 

 still in the tailed stage (Didemnum). In Botryllus, a genus which 

 is distinguished by the star-like grouping of the individuals round a 

 common cloaca (fig. 561), and by the rich branching of the blood 

 canals, the larva is simple, and does not, as Sars believed, form a 

 colony. Metschnikoff and Krohn, whose accounts agree, have both 

 shown that the eight knob-like buds of the larva are only processes 

 of the ectoderm and contain diverticula of blood spaces. The young 

 Botryllus produces only one bud (first generation), and before the 

 latter is mature perishes without attaining sexual maturity. The 

 bud of the first generation produces two buds (second generation), 

 and dies without reaching sexual maturity. The buds of the second 

 generation each produce two buds, which arrange themselves in a 

 circle, and after the death of their producers form the first system 

 with a common cloaca. In a similar manner new buds are formed, 

 and the older generation dies; the new systems are, however, as 

 transitory and are replaced by others, so that as the stock increases 

 the old generations are continually being replaced by new. In this 

 continuous process of renewal the first-formed generations have only 

 the provisional value of establishing the colony. The later generations 

 alone become sexually mature, and the female maturity is attained 

 before the male. The ova of the still young hermaphrodite gene- 

 rations are fertilized by the sperm of the older ; and it is not until 

 after the death of the latter that the testes of the former become 



