Compensatory Regulation. 67 



must, therefore, for lack of positive evidence to the contrary 

 decide that in H. uncinata the pov^er to regenerate head struc- 

 tures is found only in the thoracic region and that anterior 

 surfaces of the posterior pieces after transverse section through 

 the abdomen do not possess this power. 



The manner in which the regeneration takes place is extremely 

 interesting when compared with the development of the same 

 structures in ontogeny. It was found that in each of the cases at 

 the earliest stages three branchial buds and one opercular bud 

 were present on each side. (Fig. 23A, b, c.) This corresponds 

 with the number present in the ontogenetic development of Hydroides 

 at the first appearance of the opercula. A similar relation holds 

 for the regeneration of the branchial circlets of Apomatus after a 

 transverse cut in the thoracic region. 



A series of experiments was performed on H. pectinata to deter- 

 mine whether the cutting of the animal in two by a transverse cut 

 through the second and third segments of the thorax would cause 

 any changes in the opercula remaining at the anterior end. In 

 this series the opercula were not disturbed. The result was not 

 completely satisfactory because most of the specimens died at an 

 early stage but it was found that the cut did not cause the opercula 

 to change. A severe bodily injury, therefore, need not cause a 

 reversal. 



Another series was undertaken in the hope of finding the 

 influence of sectioning of the thorax upon the differentiation of the 

 opercula after the functional stalk had been cut at its middle. 

 It was found that the separation of the region of the body back of 

 the fourth thoracic segment from the rest does not retard the 

 changes of reversal in the opercula which usually take place after 

 a section of the functional stalk at its middle. 



A single specimen of Serpula vermicularis was operated on. 

 Both opercula were cut off, the functional one at its middle. The 

 result was a 'reversal of the former condition. The animal was 

 kept in its dish unobserved for about three months and was then 

 found to have reversed back again to its original condition. 



i. Experiments on Group IV. Apomatus ampullifera. Two 

 characteristics of the branchiae and opercula of Apomatus need to 

 be taken into account before going on with a description of the 

 experiments. (See also description, p. 26.) 



