THE DEVELOPMENT OF THE TUNICATA. 521 



V. The body of a larva as it escapes from the egir : a, the eye ; gb, the eaccular an- 

 terior end ol the central nervous apparatus into which the otolith projects ; L'g, 

 Jim, itri tubular backward prolongation ; C/is, cells of the urochord ; o, mouth; kl, 

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

 by which it coiumuuicates with the alimentary cavity ; d. commencemem of tne 

 oesophagus and stomach ; m, blood-corpuscles ; /ij), papillae by which the larva 



attaches itself. , ^ , 



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

 days old: en, endostyle; k^, branchial sac ; Ika, 2ks, nranchial stigmata ; bb. en- 

 trance into the blood-siuus between them; d, iutesiiue; 6, blood-corpudclee; klm, 

 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., Phallusia,, each e^g 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- 

 psarance. 



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

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

 sion of the eniblast, extendins* forward from the marorins 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 inv^agination closes, and an out- 

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

 which the urochord, formed bv 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 h3''poblast 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,^ The atrial tunic thus formed, 



^ * In 1852 Krohn discovered the fact tliat the larva of Phallusia is provided 

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

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



