PHYLUM CHORDATA 37 



at great depths are comparatively poorly represented, the simple 

 forms extending to a greater depth than the composite. Several 

 genera of pedunculated simple forms 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 an 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. 640), the currents which subserve respiration 

 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- 

 istics of the Craniates. 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 connection ; and further evidence is afforded by the 

 occurrence of a notochord in both, and by the similarity in the 

 development of the central part of the nervous system. But the 

 alliance is by no means a close one, and Balanoglossus and its 

 allies can only be looked upon as very remotely connected with the 

 stock from which the Urochorda are descended. 



SUB-PHYLUM III. VERTEBRATA. 



AVe have seen that the fundamental characters of the Chordata 

 are the presence of a notochord, of a dorsal hollow nervous system, 

 and of a pharynx perforated by apertures or gill-slits. In none of 

 the lower Chordata, however, are these structures found in a 



