68 MICROSCOPICAL STUDIES. 



STUDY XIII. THE TADPOLE LARV.E OF ASCIDIANS. 



The Tunicates or Ascidians, may be either fixed or free ; simple 

 or colonial. The central type of the free is seen in the Appen- 

 dicularise, transparent tadpole-like animals that sport in the surface 

 waters of our seas, in profusion, at certain seasons. In size these are 

 extremely minute, the British species averaging only i-in. to i-in. 

 in length, tail included, while the length of IJ-in. which a foreign 

 species reaches, is altogether exceptional and monstrous. In this 

 type, the tail is supported arid strengthened by a firm central rod, 

 the notochord, believed to be a similar structure to the first sup- 

 porting stiff axis of the embryos of vertebrates, and which in them 

 is subsequently usually obliterated by the encroaching growth of the 

 vertebral column. Another curious point is that two openings com- 

 municate between the pharynx and the exterior, and serve as gill 

 slits. 



These tiny ocean wanderers are never colonial, always free and 

 simple. The fixed Tunicates, the Ascidians proper, are, on the 

 contrary, very frequently colonial, or as some writers term it, com- 

 posite or compound. In such latter, there is usually a certain amount 

 of federation, such for example in those where the anal apertures 

 of a circlet of individuals open into one common atrial or cloacal 

 chamber, to be expelled by a centrally place atrial opening. 



In size, the colonial individuals are usually very much smaller 

 than the simple, but in structure the two are essentially similar, 

 having the anterior portion of the alimentary canal distended into a 

 large chamber, with walls perforated by numerous gill slits, a nervous 

 system reduced to a single rounded mass (ganglion) between mouth 

 and cloacal aperture, a blood system of much simplicity and an 

 enveloping case of nearly structureless semi-cartilaginous or gela- 

 tinous tissue, the tunic or test. 



July and August is the breeding season of many species, and if 

 one of these animals be then dissected, eggs and embryos in various 

 stages of development will be found. 



Taking one of the most advanced, the following points can be 

 observed. The body is a tiny oval, slightly flattened from side to 

 side, while from the one end is given off a long broad tail, containing 

 centrally a clear rod of a cartilaginous substance. Both the body and 

 the tail are invested in a thick coat of gelatinous material, equivalent 

 to the test of the adult fixed animal. Turning the animal on to one 

 side (when the tail is found to be now on edge) one can, by staining, 

 make out clearly the thick investing test,in which lies the darker stained 

 kernel-like body. At five points the test is broken through ; three 

 of these are where, at the anterior end, there pass an equal number of 



