ON THE DIPLOCHOliDA. 323 



further complicated by fusions. Just as in tracing down, e. g., 

 the digestive organs into lower types, we find that stages are 

 reached in which the function is more diffuse, and is confined 

 only by the extent of the hypoblast layer (indeed, not even by 

 this in Coelenterata), so in tracing down the hypoblastic 

 skeletal function we find it in the Diplochorda distributed 

 over certain areas in which it is no longer found in higher 

 types. Thus in Actinotrocha it is found in the mid-ventral 

 wall (in the form of chordoid tissue) exactly at the spot where 

 most support is required, in addition to the normal area of 

 chordoid tissue in the collar region. 



Again, in Cephalodiscus, chordoid supporting tissue is 

 developed round the pharyngeal clefts, where, again, it is 

 obvious that support is specially essential (see next {)aper). 

 It seems that in the Diplochorda the supporting function of 

 the hypoblast has been active, and chordoid tissue developed in 

 whatever position was requisite for the needs of the organism. 



The development of turgid vacuoles for support seems to be 

 the special character of the hypoblast (or endoderm), and is 

 also found in the Coelenterata, e.g. in the tentacles of 

 certain Hydroids and in some larvae, such as that of Lucer- 

 naria (R. S. Bergh). We may suppose that a feature of the 

 chordate stock is the retention of this ability to form hjpo- 

 blastic skeletal tissue by vacuolisation. 



The subneural gland is apparently to be compared with that 

 of Tunicata bearing the same name; and if we assume that 

 the line from subneural gland to nerve-ganglion marks the 

 posterior boundary of the pre-oral lobe, we may be helped in 

 the comparison of these Urochorda with other Chordata, 

 (I see no sound reason for holding that the large suboral pro- 

 cess with papilla3 for attachment, found in the larvae of sedentary 

 Ascidians, is the pre-oral lobe, but would rather regard it as an 

 hypertrophied ventral sucker belonging to the post-oral region.) 



Lastly, this organ may be compared with the hypophysis of 

 Yertebrata, and the relationship of the latter to the front 

 end of the notochord and to the subneural mesodermic tissue 

 would thereby receive a ready explanation. 



