447 



Plate XII fig. 2 of Professor Spengel's monograph, a median 

 section 0Î ScMzocardium hrasiliense, shows a shallow invagination of the 

 ectoderm of the proboscis, just inside the mouth. It is in the median 

 ventral wall of the proboscis and is exactly opposite the space behind 

 the posterior wall of the anterior body-cavity. 



Again, in Balanoglossus canadensis (PI. XVII figs. 5, 6, 7, 8) is a 

 mid- ventral invagination in the mouth- region, which squeezes in be- 

 tween the notochord and the wall of the anterior coelome (fig. 8) and 

 lies ventral to the former (fig. 5). This 'sinnes-organ' (S pen g el) has pre- 

 cisely the position and origin of the proximal part of the sub-neural 

 gland of Cephalodiscus, and appears, as far as can be judged, to be closely 

 similar in structure. Glandiceps Talahoti (PI. XIX fig. 1) appears to 

 possess an organ of a like nature, but rather less developed, and one 

 cannot help surmising that the investigation of Glandiceps ahyssicola 

 might be interesting in this connexion. 



The fact remains that in certain specie s of En ter op ne us ta a small 

 organ occurs very similar, in structure and in relative position to the 

 mouth, body-cavities and vascular system, to the lower part of the sub- 

 neural gland of Cephalodiscus whilst the proboscis vesicle presents a 

 similar likeness to the distal part of the same organ . The latter homology 

 has been emphasised in figures 1 and 2 and the former is illustrated by 

 the oblique sections shewn in figs. 3 and 4. The only important 

 diiference between the two sections is that in fig. 3 the pre- orally 

 extended notochord is shewn, (cf. PI. XVII fig. 8. Spengel.) 



Fig. 3. 



iHeart. 



Fig. 4. 



fiche Mirm. 



Fig. 5. 



■Salj-neu'il Jia«!- 



Fig. 3. Oblique section through proboscis of Balanoglossus canadensis, cutting 

 the aperture of , Sinnesorgan', (partly after Spengel.) 



Fig. 4. Oblique section through epistome of Cephalodiscus, cutting the aperture 

 of sub-neural gland. 



Fig. 5. Transverse section of pleurofthord of Actinotrocha. 



These facts alone will, I hope, be sufficient to shew that the term 

 *untenable' is not applicable to the homology suggested, I have left 



