HABITS OF ASCIDIANS. 197 



Balanoglossus, is a provisional one. In a diagram showing 

 the classification, i.e., the affinities of the different classes of 

 animals, the Tunicates may be represented as an independent 

 branch sent off possibly from the Trematode stem, running 

 parallel to the worms, near Balanoglossus, and ending a little 

 below the level of Ampliioxus. They have certainly few 

 molluscan. features, what do exist being superficial, and 

 nothing to remind us closely of the Polyzoa or Brachiopods, 

 and the class is here treated of because, in a serial arrange- 

 ment of the classes of animals, the Tunicates can best be 

 described at this place, rather than after the true worms, or 

 immediately before the vertebrates. 



The Ascidians are common just below low-water mark, 

 cither hidden in masses of mussels, attached to the rocks 

 under sea -weeds, or the compound species 

 may be found forming bright-colored masses 

 on the piles of wharves and bridges, while 

 the Botryllus grows on the leaves of eel- 

 grass. For purposes of study the larger 

 forms, such as Ascidia, Cynthia, and Bol- 

 tenia, may be dredged. On placing a good- 

 sized Ascidian in a vessel of fresh sea-water, 

 it will be found to consist of a semi-trans- 

 parent or quite opaque test with two open- 

 ings, one lower than the other, as seen in 

 Molgula, which looks, when the two orifices 

 are protruded, like a double-necked bottle. The anterior or 

 higher orifice or mouth is for the passage of currents of water 

 into the respiratory sac, and the posterior, usually lower, 

 excurrent orifice for the passage outwards of faecal matter. 

 The test is either delicate and semi-transparent, allowing the 

 intestine and ovaries and liver to be detected, or it may be 

 quite tough and opaque. It is rendered tough and hard by 

 the presence of cellulose, a substance usually occurring in 

 the walls of vegetable cells, but very rarely in animal tis- 

 sues. 



Order 1. Ascidiacea. As an example of the ordinary Tuni- 

 cates, we will now study the internal anatomy of Boltenia 

 reniformis (Fig. 136), which lives in from five to twenty-five 



