HABITS, ETC., OF OEREBRATULUS LACTEUS. 119 



There are also well-developed dorso-ventral muscles in the 

 walls separating the intestinal casca from the reproductive 

 poucheSj and these muscles are enlarged into thick pillars at 

 the inner ends of the walls (fig. 60). Finally, an examination 

 of the regenerating longitudinal muscles shows that in early- 

 stages of growth they consist of both longitudinal and trans- 

 verse fibres, about equally divided (fig. 60). Later the longi- 

 tudinal fibres become more numerous, but there are always 

 many transverse ones to be found among them. 



We are left to conclude that in Cerebratulus dismember- 

 ment is at least greatly assisted, if it be not entirely produced, 

 by a sudden and excessive contraction of these transverse 

 muscles of the body. The dorso-ventral muscles being meta- 

 merically arranged, and the others being evenly distributed, 

 dismemberment would occur at a definite place in each seg- 

 ment. 



The final question is one which demands the method of re- 

 generation ; we shall note only the most important points. 

 Immediately after the rupture is completed the walls of the 

 body are drawn together, and held in that position by the 

 contracted circular muscles. This brings the severed edges 

 of the ectoderm into contact with each other, and they 

 quickly unite and heal the wound over with the exception of 

 the posterior end of the intestine, which is left open to func- 

 tion as an anus. The cells in this newly formed ectoderm, 

 and the mesoderm cells immediately beneath them, proliferate 

 rapidly and form a regenerating papilla which grows back- 

 ward, keeping the anus open at its extremity. This papilla 

 is much paler in colour than the rest of the body, and at first 

 consists of a thin ectoderm entirely filled except at the centre 

 with mesoderm cells. At the same time the entoderm lining 

 the intestine grows backward as a straight tube along the 

 centre of the papilla. The muscular system is, of course, 

 developed from the mesoderm, and two things are worthy of 

 note in its differentiation. The circular muscles appear first, 

 and are followed by the outer longitudinal layer. The latter 

 is developed very slowly, and for a long time consists of a 



