THE DEVELOPMENT OF THE TUNICATA. 521 



V. The body of a larva as it. escapes from the eg<: : a, the eye ; gb, the saccular an- 

 ' terior end of the central nervous apparatus into which the otolith projects ; Eg, 



Em its tubular backward prolongation; Chs, cells of the urochord ; o, mouth; kl, 

 atrial aperture ; /, opening at the anterior end of the central nervous apparatus, 

 by which it communicates with the alimentary cavity ; d, commencement of the 

 oesophagus and stomach ; m, blood-corpuscles ; lip, papillae by whicii the larva 

 attaches itself. 



VI. The body and the commencement of the caudal appendage of a tree larva two 

 days old • en, endostyle ; ks, branchial sac ; Iks, 2ks, branchial stigmata ; bb. en- 

 trance into the blood-siuus between them; d, intestine; b, blood-corpuscles; Mm, 

 atrial aperture. 



The reproductive organs of the two sexes are united. 

 Usually, the testis and the ovary have the form of racemose 

 glands situated in the loop formed by the intestine ; or be- 

 yond it, when the "abdomen" is long; and their ducts run 

 parallel with one another, to open close together beside the 

 anus. In many of the simple Ascidians, however, the repro- 

 ductive organs' are lodged in the lateral walls of the atrial 

 cavity, and their ducts are distant from the anus; and, some- 

 times, there are many distinct genital glands. 



In some genera, e. g., JP/iallusia, each egg is surrounded 

 by an ovicapsule, formed by the coalescence of cells of the 

 epithelial lining of the ovary, and these cells may grow out 

 into processes which give the fully-formed egg a stellate ap- 

 pearance. 



Complete yelk-division takes place, and the morula under- 

 goes invagination (Fig. 149, I., II.). A longitudinal depres- 

 sion of the epiblast, extending forward from the margins of 

 the aperture of invagination, next makes its appearance ; 

 and, deepening, gives rise to an involution, the edges of 

 which unite, and thus shut off a tubular portion of the epi- 

 blast. This is the rudiment of the nervous ganglion (Fig. 

 149, III.). The aperture of invagination closes, and an out- 

 growth of the body gives rise to the caudal appendage, into 

 which the urochord, formed by the coalescence of certain cells 

 of the hypoblast, extends (Fig. 149, IV.). The sac of the 

 hypoblast becomes divided into its branchial, oesophageal, 

 gastric, and intestinal portions, and the mouth is formed by 

 the perforation of a spot in which the hypoblast and the epi- 

 blast cohere (Fig. 149, VI.). The atrial cavity is formed by 

 two involutions of the ectoderm, which extend inward and 

 apply themselves to the lateral and neural walls of the bran- 

 chial sac (Fig. 149, VI.). Their originally separate apertures 

 eventually coalesce into one. 1 The atrial tunic thus formed, 



1 In 1852 Krohn discovered the fact that the larva of Pliallusia is provided 

 with two distinct symmetrically-disposed openings, by which the originally 

 separate atria open outward ; and that the two eventually coalesce into the sin- 



