FORM REGULATION IN CERIANTHUS /ESTUARII. 4 1 



ant than that of C. astuavii and is capable of supporting without 

 the aid of the tube, a degree of internal pressure which closely 

 approaches or perhaps equals the normal. In that species, there- 

 fore no marked reduction of the tentacles is observed in specimens 

 kept without sand. 



IV. Lateral Partial Discs. 

 The formation of lateral partial discs occurs in connection with 

 lateral incisions and usually in the oesophageal region only, 

 where the cut edges of the body-wall and the oesophagus fuse 

 both above and below the cut and so give rise to a lateral mouth 

 (Child, 'o5«). In all cases observed in C. solitariiis the region 

 between the lateral cut and the oral end of the body undergoes 

 complete atrophy sooner or later and the lateral partial disc takes 

 the place of the atrophied part. Atrophy occurs in this region 

 because its enteric cavity has no connection either with the other 

 portions of the enteron or with the exterior. After closure some 

 distension occurs but collapse follows and continues until the 

 part undergoes complete atrophy. Meanwhile the partial disc 

 gradually changes its position toward the oral end and finally 

 replaces the atrophied part. 



{a) Transverse Lateral Incisions. 

 The only difference worthy of note in this connection between 

 C. solitarins and C. (ESttiarii is the more rapid atrophy of the 

 portion oral to the cut in the latter species. Frequently this 

 atrophy is so rapid that breaks occur in the piece within a week 

 or ten days after the operation, while in C. solitariiis this condi- 

 tion is reached only after several weeks. Fig. 14 shows a longi- 

 tudinal section of a specimen with lateral partial disc and the 

 collapsed portion undergoing atrophy oral to it. The incision in 

 this case was made at a, Fig. 13. 



{b) Oblique Lateral Incisions Directed Aborally. 

 No oblique lateral incisions were made in my earlier work on 

 C. solitarins, but a considerable number of operations of this kind 

 have been made upon C. astuarii. When the incision is directed 

 aborally at any angle with the transverse plane up to 5o°-6o° 

 {b. Fig. 13) partial discs appear which show the difference in 



