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 at 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 Balanoglossus, and this structure in 

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

 ryngeal or gill-sac of Amphioxus and vertebrate embryos. 



The simple Ascidians attain to a large size, Ascidia callosa 

 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 Pyrosoma 

 gigas is sometimes five feet long. Cynthia pyriformis 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 Hypolythius calycodes 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 lythius 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 Amarwcium, etc., where the individuals of 

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

 the larval condition and has become fixed. In Didemnium 

 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 Botryllus the zooid which 

 results from the tadpole -like larva serves, according to 



