258 BULLETIN 100, UNITED STATES NATIONAL MUSEUM. 



width 1.8 mm. to 2.5 mm. The cloaca is long, in this form, usually 

 about twor-thirds the length of the branchial sac, which is 3 mm. to 

 3.5 mm. long. The cloaca is often found to be quadrangular in cross- 

 section. There is extreme diversity as to the size and shape of the oral 

 chamber, but it takes the form of a long, slender siphon in the ma- 

 jority of cases. Two circum-oral fibres can be seen, not always 

 reaching completely around the oral chamber; the anterior one is 

 sometimes abortive or very much constricted. 



The luminous organs in this form are large and distinct. They 

 are circular or quadrangular, but vary greatly in size, even those on 

 the two sides of the same zooid being of different sizes in some in- 

 stances. In each branchial wall, which is long and elliptical, there 

 are 16-17 longitudinal bars and 30-36 stigmata! rows. The endo- 

 style never describes a very strong curve. The dorsal languets num- 

 ber from 8 to 9. The digestive tract is small, but in other respects 

 is as usual for the species atlanticum. 



The testis consists of a large number of lobes, 20-26, some of which 

 may again be partially divided into secondary lobes or pouches. The 

 entire organ becomes large and prominent when mature. We find 

 the condition of protogym^ almost universal among the zooids of 

 this one colony. Perhaps smaller ones would be found to contain 

 a greater proportion of protandrous individuals. 



Our knowledge of the occurrence of this form is based upon the 

 single specimen obtained from the Naples Zoological Station. It is 

 probable that it was secured in the Bay of Naples, or in the Mediter- 

 ranean Sea, but its label bears no statement of the place of collec- 

 tion. This specimen has been deposited in the United States 

 National Museum, and bears Cat. No. 6437 (Type) U.S.N. M. 



GENERAL CONSIDERATIONS. 



RELATIONSHIPS WITHIN THE FAMILY. 



The accompanying chart indicates some possible relationships 

 among the Pyrosomata ambulata, and also their apparent centers of 

 distribution. The subspecies atlanticum, seems to be the parent 

 form for the whole atlanticum group. Its home is apparently the 

 Pacific Ocean. The smooth forms are shown as a distinct group, 

 some members of which, however, interbreed with the atlanticum 

 group ; thus, we have four colonies which seem to be hybrids between 

 P. atlanticum atlanticum and P. verticillatum, subspecies cylindricum. 1 

 If the form dipleurosoma is to be regarded as a distinct subspecies 

 of atlanticum, we would have to say, from the evidence in our col- 

 lections, that this subspecies interbreeds with atlanticum atlanticum 2 



i United States National Museum, Cat. Nos. 6US (type) and 6417. 

 *Idem, Cat. Nos. 0471 (type) and 6421. 



