

DEVELOPMENT OF THE ASCIDIANS. 505 



the " solitary" Ascidians ; and also in those composite forms, 

 whose connecting stolons do not contain bloodvessels. 



335. The development of the Ascidians presents some pheno- 

 mena of much interest to the Microscopist ; the early stages of 

 which are observable, whilst the ova are still within the cloaca 

 of the parent. After the ordinary repeated segmentation of the 

 yolk, whereby a " mulberry mass," is produced, a sort of ring is 

 seen, encircling its central portion ; but this soon shows itself as 

 a tapering tail-like prolongation from one side of the yolk, which 

 gradually becomes more and more detached from it, save at the 

 part from which it springs. Either whilst the egg is still within 

 the cloaca, or soon after it has escaped from the vent, its en- 

 velope bursts, and the larva escapes ; and in this condition it 

 presents very much the appearance of a tadpole, the tail being 

 straightened out, and propelling the body freely through the 

 water by its lateral strokes. The centre of the body is occupied 

 by a mass of liquid yolk ; and this is continued into the interior 

 of three prolongations which extend themselves from the oppo- 

 site extremity, each terminating in a sort of sucker. After swim- 

 ming about for some hours with an active wriggling movement, 

 the larva attaches itself to some solid body by means of one of 

 these suckers; if disturbed from its position, it at first swims 

 about as before ; but it soon completely loses its activity, and be- 

 comes permanently attached ; and important changes manifest 

 themselves in its interior. The prolongations of the central yolk- 

 substance into the anterior processes and tail are gradually drawn 

 back, so that the whole of it is concentrated into one mass ; and 

 the tail, now consisting only of the gelatinous envelope, is either 

 detached entire from the body by the contraction of the connect- 

 ing portion, or withers and is thrown off gradually in shreds. 

 The shaping of the internal organs out of the yolk-mass, takes 

 place very rapidly ; so that by the end of the second day of the 

 sedentary state, the outlines of the branchial sac and of the sto- 

 mach and intestine may be traced ; no external orifices, however, 

 being as yet visible. The pulsation of the heart is first seen on 

 the third day, and the formation of the branchial and anal orifices 

 takes place on the fourth ; after which the ciliary currents are 

 immediately established through the branchial sac and alimentary 

 canal. The embryonic development of other Ascidians, solitary 

 as well as composite, takes place on a plan essentially the same 

 as the foregoing, a free tadpole-like larva being always produced 

 in the first instance ; and in the curious Appendicularia, which 

 occasionally presents itself on our own coasts, this larval form is 

 retained through life. 1 



' For more special information respecting the Compound Ascidians. see especially 

 the admirable Monograph of Prof. Milne Edwards on that group, Mr. Lister's Memoir 

 " On the Structure and Functions of Tubular and Cellular Polypi, and of Ascidiae," in 

 the " Philos. Transact." 1834, and Mr. Huxley's Memoir " On Doliolum and Appendicu- 

 laria," in "Philos. Transact/' 1851 ; also the Art. Tunicata in the "Cyclopaedia of Ana- 

 tomy and Physiology. ' ; 



