68 Charles ZeJeny. 



In the first place there are two opercula, one a large spherical 

 body and the other a very small terminal enlargement, each at the 

 end of the branchial stalk occupying the next to the dorsal position 

 in its branchial circlet. This stalk is in each case a typical 

 branchia except for the opercular enlargement and apparently 

 carries on its iull respiratory as well as its opercular function. 

 (Fig. 23D.) 



In the second place each branchial semicirclet taken as a whole 

 breaks off very readily along a definite line at its base so that all the 

 branchiae including the opercular one come off together. Thus a 

 very slight irritation is sufficient to cause the animal to throw off 

 the whole branchial apparatus, including the opercula. The 

 fission plane is in a very definite region at the base of the branchial 

 circlet and after coming off the whole branchial crown holds 

 together in one piece because of the union of the branchiae near 

 their bases. The right and left branchial circlets act independ- 

 ently in the matter since it often happens that only one is cast off. 

 Usually, however, both are thrown off. Such an operation as the 

 removal of the animal from its tube usually brings about this 

 autotomy of the branchiae. Out of 42 specimens removed from 

 their tubes on November 6, 1902, thirty lost both branchial circlets, 

 6 lost one of the circlets and only 6 retained the whole branchial 

 crown. For this reason it w^as not possible to repeat the ordinary 

 operculum reversal expermients on Apomatus as after such an 

 operation the branchial circlet was cast off. 



After the casting off of the branchial crowns in these cases a 

 regeneration of two functional opercula usually followed, though 

 one was often larger than the other. Probably correlated with the 

 differentiation of a "breaking joint" at the base of the circlet is 

 the fact that the regeneration of the branchial crown does not 

 show only three branchiae plus the operculum at the first differen- 

 tiation as in Hydroides but at once brings out several branchiae on 

 each side. One of these may show the opercular differentiation 

 from the start, while in other cases it develops first as a branchia 

 and only later shows the opercular character. 



The regeneration at the anterior end of a posterior piece after 

 transverse section through the thoracic region showed a less highly 

 differentiated character of the new organ at the start than when 

 the regeneration took place from the "breaking joint." A great 

 number of operations were made, but the anmial proved very 



