448 



Mr. A. G. Huntsman. 



[Mar. 3, 



The number of protostigmata is indefinite. Damas gives more than 15, 

 Julin 12 at least. In the Botryllidee the number is smaller ; from 5 to 7 

 have been observed. Their method of formation has not been established. 

 The majority appear about the time of metamorphosis, when observation is 

 difficult. I can affirm independent perforation for the sixth in a species of 

 Botryllus from Naples. The others in size show no evidence of intercalation. 



In the Ccesiridfe we have in many respects the other extreme. The 

 number is definite and intercalation is regular. Three stigmata perforate in 

 the usual order and each one by turning upward at the ventral end and 

 dividing gives rise to a second placed immediately behind it, as is shown in 

 the first series of stages in the figure. Six protostigmata arranged in three 

 pairs are thus formed. The same method is followed by Pyura haustor, one 

 of the Tethyidse. I have observed in this species the derivation of the 

 second and fourth from the first and third, but not of the sixth from the 

 fifth. Only six protostigmata are formed and their first subdivision to form 

 the definitive stigmata is the same as that which has been described for the 

 Ceesiridae. 



A slight modification of this method is to be found in another Tethyid, 

 Tethyum [Cynthia] pyriforme. The turned up portion of the stigma is cut 

 off very early, probably before the opening of the stigma has extended into 

 it. I have not observed all the stages but can affirm that the second is 

 intercalated from the first and that only six protostigmata are formed. Also 

 in the earliest stage examined only one stigma is present. 



A further modification occurs in the genera Boltenia (B. ovifera and 

 B. villosa) and Styela (S. gibbsii) of which I have examined a full series of 

 stages. The second and fourth are formed by the abstraction of the ventral 

 growing tips of the first and third. Subsequently these perforate and 

 extend dorsoventrally. The fifth and all succeeding protostigmata arise in 

 order by independent perforation at points along the middle of the side of 

 the pharynx. They give no indication of being derived from pre-existing 

 ones. Their number is indefinite. I have observed stages with 11 in 

 Styela and with 9 in Boltenia. The two genera differ greatly in the 

 method by which the definitive stigmata are formed from the protostigmata. 



The method just described for these two genera is an approach to the 

 method of Dendrocloa and the Botryllidas. The third primary protostigma 

 has lost its ability to form a new one by intercalation. If the first and 

 second likewise lost that power we would have the condition in Dendrodoa. 



All the forms so far considered belong to the Order Ptychobranchia. 

 This group is therefore characterised by having three or more primary 

 protostigmata, which perforate in regular order. Secondary protostigmata may 



