42 ZOOLOGY SECT. 



formed embryo, in other words, gives rise to an asexual genera- 

 tion, which escapes to the exterior and becomes free-swimming 

 (Fig. 727). After a time there is developed a process or stolon 

 (stol.), on the surface of which are formed a number of bud-like 

 projections. These increase in size as the stolon elongates, and 

 each eventually assumes the form of a sexual Salpa. The stolon 

 with the Salpse attached becomes separated off and swims about 

 as a chain of zooids in which the reproductive organs are developed. 



Distribution, etc. The pelagic forms are, as is the case with 

 most pelagic organisms, of very wide distribution, and none of the 

 genera are confined to particular oceanic areas. The fixed forms, 

 both simple and composite, are also of world-wide distribution ; 

 they are much more abundant in the southern hemisphere than in 

 the northern the composite forms attaining their maximum in 

 the South Pacific area. The depth to which the pelagic forms 

 extend has not been determined. Fixed forms occur at all depths, 

 but are much more numerous in shallow water than in deep, and 

 at great depths are comparatively poorly represented, the simple 

 forms extending to a greater depth than the composite. Several 

 genera of pedunculated simple Ascidians seem to be confined to 

 very great depths. 



Though placed so high in the animal series, the Urochorda 

 exhibit very low functional development. This is chiefly connected 

 with the sessile condition of most of them. The movements per- 

 formed by a fixed Ascidian are slow and very limited in character, 

 being confined to contractions of the mantle; when the animal is 

 detached, such contractions may be sometimes observed to result 

 in a slow creeping locomotion. Even in the free forms the move- 

 ments are limited to the contractions, of the tail muscles in 

 Appendicularia, of the muscle-bands of the body-wall in Doliolum, 

 by which swimming is effected. The mode of obtaining food 

 resembles that which has already been described in the case of the 

 Pelecypoda (Vol. I., p. 689), the currents which subserve respira- 

 tion also bringing in microscopic organic particles to the mouth. 



Affinities. That the Urochorda are degenerate descendants of 

 primitive Chordates admits of little doubt; the history of the 

 development of the Ascidians, taken in connection with the occur- 

 rence of permanently chordate members of the group (Appendicu- 

 laria and its allies), is quite sufficient to point to this conclusion. 

 But the degree of degeneration which the class has undergone 

 the point in the line of development of the higher Chordata from 

 which it diverged is open to question. According to one view 

 the Urochorda are all extremely degenerate, and have descended 

 from ancestors which had all the leading features of the Craniata ; 

 according to another, the ancestors of the class were much lower 

 than any existing Craniate lower in the scale than even Am- 

 phioxus and had not yet acquired the distinctive higher character- 



