xm PHYLUM CHORDATA 41 



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 sometimes be 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. 671), 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 Amphioxns 

 and had not yet acquired the distinctive higher characteristics 

 of the Craniates. The complete want of segmentation and the 

 virtual absence of a coelome seem to point in the latter direction : 

 the presence in the larva of highly-developed central nervous 

 system and sense-organs in the former. Appendicularia is hardly 

 to be regarded as representing a primitive ancestral type ; its 

 close resemblance to the larva of the sessile Ascidians rather seems 

 to indicate that it is a persistent larval form a form in which 

 sexual maturity has been reached at earlier and earlier stages in 

 the life-history, and in which the final sessile stage has at last been 

 lost, the animal having become completely adapted to a pelagic 

 life. Probably the other pelagic forms Salpa, Doliolum, Pyrosoma 

 were also descended from sedentary ancestors : none of them 

 shows any character that can be interpreted as primitive. 



The nearest existing ally of the Urochorda among lower forms 

 is probably Balanoglossus. The similarity in the character of 

 the pharynx, or anterior segment of the enteric canal, perforated 

 by branchial apertures, is alone sufficient to point to such a connec- 

 tion ; and further evidence is afforded by the occurrence of a 

 " notochord " in both, and by the similarity in the development 



