TUNICATA. 481 



Eysenhardt* has observed the act of gemmation in a simple As- 

 cidian. In this, as in the compound kinds, gemmation commences 

 by the development of a small tubercle from the abdominal portion of 

 the internal tunic of the young Ascidian. This is prolonged, retain- 

 ing an active circulation in its interior, and is accompanied by a cor- 

 responding growth of the outer gelatinous integument, which becomes 

 clavate. The process then bifurcates ; the divisions, in like manner, 

 becoming elongated, expanded, and bifurcated at their extremities. 

 Soon the outline of an Ascidian is sketched in each of these extremi- 

 ties. The primitive connection with the parent is obliterated ; but 

 the young individuals remain united together by their common 

 peduncle, according to the law which determines their mode of 

 grouping into systems. By the progressive increase of their outer 

 gelatinous integument they coalesce and form the compound mass. 



The procreative force of the germ-mass finally exhausts itself in 

 the formation of the male and female organs ; in which that force is, 

 again, mysteriously renewed, under its two forms of the sperma- 

 tozoon and the germinal vesicle, by the combination of which the 

 reproductive cycle again begins its course. On comparing this course 

 of development with that in the Bryozoa, p. 151, I may again request 

 attention to the genetic difference, in addition to the absence of the crown 

 of radiating tentacles and the presence of the vascular respiratory sac, 

 which distinguishes the compound Ascidians from the Bryozoa, and 

 which seems to have been overlooked by those who w^ould call the 

 latter MoUusks. No compound Ascidian quits the ovum as a ciliated 

 gemmule, swimming by means of groups of those vibratile organs 

 aggregated in lobes, after the type of the Rotifera ; and no Bryo- 

 zoon, so far as I know, quits the ovum in the guise of a tadpole or 

 cercarian, swimming by the alternate inflections of a caudal ap- 

 pendage. 



If the term Appendicularia should not prove to have been applied 

 to a caudate larve of some fixed species, that genus will permanently 

 represent the transitional locomotive stage of the rest of the Asci- 

 dians. Its organisation does not, however, lead to the next division 

 of the class. 



A connecting link between the Ascidians and Salpians is 

 afforded by certain compound floating gelatinous Tunicata, e. cj. 

 the phosphorescent Pyrosoma, the individuals of which are per- 

 manently aggregated into a compound organic whole having a 

 definite form, like a flattened cylinder. The common tegumentnry 



♦ CCXCVIII. p. 263. t. 30. fig. 

 I I 



