446 ZOOLOGY 
and often by the presence of ova in it; the vas deferens is 
a very narrow duct of an opaque white colour. 
The structure of an adult Ascidian, such as Ciona, shows 
but slight resemblance to that of the more typical Chordata, 
but if the life-history of one of these creatures be followed out, 
it is seen that the larval stages are much more highly differ- 
entiated than the adult, and that in all essentials they conform 
to the type of the Chordata. The free-swimming tadpole 
Fic. 260.—Stages in the embryology of a simple Ascidian. After Kowalevsky. 
A. Embryo showing body and tail and ep. Epiblast. 
completely formed neural canal. hy. Hypoblast. 
B. Larva just hatched: the tail is cut ne. Neural canal. 
off. oc. Ocular organ of larva. 
C. Transverse section of tail of larva. m. Muscle cells of tail. 
adp. Adhering papillae of larva. mes. Mesenteron. 
at. Epiblastic atrial involution. mc. Mesoderm cell, 
au. Auditory organ of larva. nv. Cerebral vesicle at anterior end of 
ch. Notochord, neural canal. 
larva of the simple Ascidians is provided with a tail, in which 
is a Skeletal rod of supporting tissue, the notochord. Unlike 
the same structure in other Chordata, this rod is confined to 
the tail. Above this hes the tubular nervous system, which 
gives off several pairs of nerves to the muscles of the tail; these 
latter show some traces of metameric repetition. Anteriorly 
the nervous system is enlarged, and a median eye and auditory 
sac are connected with it. The origin of the various organs 
in the larva closely resembles that of the other Chordata. 
The gill-slits in the larva do not exceed a few pairs in 
number, and it has been maintained that the numerous 
stigmata opening through the walls of the branchial sacs 
