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 dona, shows 

 but slight resemblance to that of the more typical Ohordata, 

 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 



Fie. 260. — Stages in the embryology of a simple Ascidian. After Kowalevsky. 



A. Embryo sbowing body and tail and 



completely formed neural canal. 



B. Larva just hatched : the tail is cut 



oflf. 

 C. Transverse section of tail of larva. 

 adp. Adhering papillae of larva. 

 at. Epiblastic atrial involution. 

 au. Auditory organ of larva. 

 ch. Notochord. 



ep. Bpiblast. 

 hy. Hypoblast. 

 nc. Neural canal. 

 oc. Ocular organ of larva. 

 m. Muscle cells of tail. 

 mes. Mesenteron. 

 mc. Mesoderm cell. 



nv. Cerebral vesicle at anterior end of 

 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 lies 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-sHts 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 



