CHAPTER 13 



EYES OF INTERMEDIATE TYPES BETWEEN 

 INVERTEBRATES AND VERTEBRATES 



These, commonly known as Protochordata, are classed in three Sub- 

 phyla : 



(1) Hemichorda, including Balanoglossus, Fig. 123. 



(2) Urochorda, comprising the tunicates (Ascidia or sea-squirts) ; 



Salpa (Fig. 128, A, Chap. 14, p. 176) ; Doliolum and allied 

 forms, many of which are fixed and form colonies such as 

 Pyrosoma. 



(3) Euchorda (Acrania or Cephalochorda), represented by Amphioxus 



or lancelet (Fig. 129, Chap. 16, p. 179). 



All these classes are interesting with respect to the eyes which are 

 present in the larval stages of certain representative types, and from 

 the general standpoint of morphology although they are not in the main 

 line of descent of vertebrates and are to be regarded as side-branches some 

 of which have undergone regressive changes. 



There is still considerable difference of opinion with regard to the 

 affinities of each of the three groups enumerated above, which are some- 

 times described together under the general term Protochordata. The 

 disagreement has been specially concerned with the relationships of the 

 Subphylum Enteropneusta, or Hemichorda, which includes such apparently 

 different species, as the free-swimming Balanoglossus and the fixed type 

 Cephalodiscus. The affinity of these with the vertebrates was at one time 

 definitely denied by many morphologists, although the Urochorda or 

 tunicates have for a long time been recognized as degenerate vertebrates 

 which have undergone adaptational changes, such as a colonial or a fixed 

 condition of life in the adult animal which is similar to that of many 

 protozoa or coelenterates and aquatic plants. 



The very close resemblance of the tornaria larvae of the Hemichorda 

 (Fig. 125, Chap. 14, p. 172) to the trochophore larvae of invertebrates, 

 more particularly of the annelids and the echinoderms — a resemblance 

 which includes such likenesses as are found in the apical plate, the 

 cerebral ganglia, the two ocelli ; the disposition of the ciliated bands and 

 apical tuft of hairs (Fig. 124) ; also of the mouth, alimentary canal, and 

 anus — led certain authors such as Spengel to challenge the earlier con- 



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