392 ZOOLOGY 



metrical relation of parts seen in the true worms, and which 

 strongly suggest the conclusion that the Tunicates are mod- 

 ified worms. This conclusion is strengthened by the fact 

 that in Appendicularia the ventral nervous cord is gangli. 

 onated yt intervals, as in the Annelids, while the twisted 

 digestive tract is much as seen in Polyzoa and Brachiopods. 

 Furthermore, the branchial sac is strongly analogous to the 

 pharyngeal or gill-sac of B:ila)io<jlossus, and this structure in 

 the Ascidian and whale's-tongue worm anticipates the pha- 

 ryngeal or gill-sac of AtnpMoxus and vertebrate embryos. 



The simple Ascidians attain to a large size, Ascidia callo^a 

 being about ten centimetres in diameter, quite round, and in 

 form and color bears a strong resemblance to a potato. 

 Ascidia gigas, dredged by the Challenger Expedition, is from 

 thirty to forty centimetres in diameter, and has a ganglion 

 nearly as large as a pea. A floating colony of Pyroxoma 

 gigas is sometimes five feet long. Cyutlt ia pyrifonn /'x Rathke 

 may be called the sea-peach, from its size, form, and the rich 

 bloom and reddish tints of its test. It is common in deep 

 water from Cape Cod to Greenland and Scandinavia. 



While the Ascidians as a rule do not live below a depth of 

 150 fathoms, the stalked Hypobythius call/codes Moseley was 

 dredged by the Challenger Expedition in 2900 fathoms in 

 the North Pacific Ocean ; it is stalked, and about twenty 

 inches high. The aberrant Octacnemus bythius Moseley was 

 also dredged in 1070 fathoms near the Schouten Islands, 

 Tasmania. 



Panceri has described the luminous organs of Pyrosoma, 

 which is highly phosphorescent ; the substance from which 

 the light is emitted is probably a fatty matter. 



Ascidians multiply by budding and by eggs. Examples of 

 budding or germination are seen in the compound or social 

 Ascidians, such as Amarcecium, etc., where the individuals of 

 the colony bud out from the primitive one just as it lias left 

 the larval condition and has become fixed. In Dideniniuni 

 buds arise from masses of cells floating free within the test. 

 They multiply by division as soon as the digestive and repro- 

 ductive organs are indicated. In Botryllns the zooid which 

 results from the tadpole -like larva serves, according to 



