Compensatory Regulation. 4 1 



the two opercula at their first appearance and up to the time when 

 they assume the final adult condition. 



The following observations on H. dianthus were made at the 

 Cold Spring Harbor Biological Laboratory on Long Island, N. Y., 

 during July, August and September, 1902. The observations on 

 the other species, H. uncinata and H. pectinata, were made at the 

 Naples Zoological Station in the winter of 1902-03. The obser- 

 vations on the method of rearing the larvae and on their general 

 activities will be given in a separate short note.^ 



c. Observations. When the free-swimming larva is about nine 

 days old its body is considerably elongated and shows external signs 

 of segmentation. The apical cilium is long and two very promi- 

 nent, reddish eyespots are present. (Fig. iga.) As the move- 

 ments of the animal become more and more sluggish just before 

 its fixation to a solid object the apical cilium gradually grows 

 smaller until it disappears entirely. At the same time three pairs 

 of setae are formed, the first pair being especially long and promi- 

 nent. (Fig. lOB.) Fig. IOC shows a larva, 17 days old, with the 

 tube covering about one-half of the body.^ Here each of the two 

 lateral head lobes already shows the division into three blunt pro- 

 cesses, the forerunners of the branchiae. These divisions of the 

 head lobe appear very soon after the formation of the head lobe 

 itself but the latter has a short separate existence before the 

 tripartite subdivision appears. The two dorsal processes are 

 more closely connected together than with the third and more 

 ventral one. The relation is more clearly made out in Fig. iod, 

 where the mutual union of the two dorsal pairs is very evident. 

 The two larvae (Fig. loc and Fig. iod) are of the same age (17 

 days) and come from the same dish. 



In Fig. IDE there are still the same three pairs of processes 

 though the branchial character is more evident than before. 

 This specimen is from the same dish as the others (16 days after 

 fertilization). It is evident from these data that the rate of de- 

 velopment of the diff^erent larvae in a single dish varies within 

 wide limits. At the stage represented in Fig. ice the inner sur- 

 faces of the branchiae are covered with very active cilia. In order 



'Biological Bulletin, '05, vol. viii. 



2The tube secreted by the animal is at first a very short cylinder which is quite transparent and covers 

 only a small part of the larva. The ring at first is situated near the anterior end of the body just back 

 of the eyes. 



