358 TUNIC ATA. 



in the common Ascidia. In others, budding has resulted 

 not in mere clusters, but in composite organisms, e.g., 

 Botryllus. Furthermore, the unitedness of the colony may 

 be very thorough, as in the fire-flame Pyrosoma, which, though 

 composed of numerous individuals, swims as one creature. 



Very different from the above, none of which are very far 

 removed from the common Ascidian type, are the free- 

 swimming genera Salpa and Doliolum. They have a com- 

 plex structure, and exhibit alternation of generations. 



Finally, there are a few genera which retain the larval 

 characteristics. Of these, Appendicularia is the simplest type. 



(I.) — The Appendicularia Type. 

 Sub-Class Larvacea (synonyms Copelata, Perennichordata). 



Brief description of Appendicularia. — It is instructive to begin with this 

 genus, for it retains the larval characteristics. The animals are free- 

 swimming and minute. Their shape is somewhat like that of larval 

 Ascidians. A long locomotor tail projects ventrally beneath the 

 small oval body. Epidermic cells near the mouth secrete a slimy but 

 consistent test, or " house," which is abandoned and formed anew from 

 time to time. The tail with its supporting notochord shows hints of 

 segmentation or vertebration. The nervous system consists of a lobed 

 ganglionic mass above the mouth, of a nerve-cord connecting this with 

 a second ganglion at the beginning of the tail, along which (rather to 

 one side) the nerve-cord is continued with more ganglia. Both cord and 

 ganglia are said to include an axial canal, and nerves issue from the 

 ganglia. The cerebral ganglionic mass has in connection with it a 

 pigment spot (optic?), an otocyst (auditory?), and an inferior tubular 

 process communicating with the pharynx. There are only two gill- 

 slits. The mouth is almost at the anterior end ; the food-canal ends 

 at the root of the tail. The heart is ventral, without definite vessels. 

 The hermaphrodite reproductive organs lie posteriorly and are ductless ; 

 there is a special dorsal brood-pouch. 



Genera. — Appendicularia. Oikopleura. Fritillaria. Kowalevsliia. 



(II.) — Ascidia Type. 

 Sub-Class Ascidiacea. 

 Description of a simple Ascidian. — To common sea-squirts 

 like Ascidia, Phallusia, Ciona, the following description will 

 apply. 



The shape is more or less that of a double-mouthed flask ; 

 mouth and exhalent aperture lie near one another. 



The epidermis secretes a cuticle or tunic, which in part 



