252 BULLETIN 100, UNITED STATES NATIONAL MUSEUM 



on which the operculum is supported, but they are almost always 

 invisible exteriorily. 



Tentacles. — There are about 25 tentacles in P. (Lepralia) japonica. 

 "Sections show that two of the tentacles are larger than the others, 

 extending be3 7 ond them and having larger nuclei. This is the case in 

 a large number of species." (Waters, 1907.) 



"In the species of this group which it has been possible to examine 

 there is a large number of tentacles, namely, Petralia undata Mac- 

 Gillivray, 26; P. japonica Busk, 25; P. castanea, 23; P. vultur var. 

 armata Waters, 25; P. chuakensis Waters, 25 (Waters, 1913). 



"In Petralia imltur var. armata, there seem to be two tentacles larger 

 than the rest (figs. 15-17), and these, instead of being triangular, 

 have the inner surface nearly straight with a number of long nuclei. 

 On this inner surface there are cilia, but unfortunately the condition 

 does not admit of exact study of this point. These two larger ten- 

 tacles occur in all the species examined of the group and larger ten- 

 tacles occur in other groups to which I have previously alluded. 

 These large tentacles are most marked near the base of the tentacles 

 when they are commencing to divide, whereas -nearer to the ends there 

 is but little difference." (Waters, 1913.) 



Shield. — "In one respect Lepralia dorsiporosa appears to me to show 

 more primitive characters than L. pallasiana. The distal prolonga- 

 tions (/. sh. d.) of the frontal shield do not completely surround the 

 orifice, the point where they meet being commonly indicated by a 

 slight emargination on the distal side of the zooecium. Each distal 

 prolongation typically bears an avicularium (avic)." (Harmer, 1902.) 

 The presence of the distal shield on each zooecium is a veritable 

 character of the family. Like all calcareous secretions it is very 

 variable and appears to be a special adaptation to the biological 

 conditions of the surrounding medium. It is often bipartite. The 

 distal portion and its lateral margins are much reduced when it bears 

 under the aperture the avicularian umbo. It is a piece juxtaposed to 

 the tremocyst and superposed on the subjacent olocyst. Rarely it is 

 partially hidden by the tremocyst and its limits are perfectly marked. 

 In the description of species we have occasion to indicate the variations. 



Radicles. — "With few exceptions the colonies are free, one-layered, 

 laminate, and in such cases the basal wall of the colony is provided 

 either with numerous pores or more frequently with one or a few pore 

 chambers placed at the distal end from which radical fibers sometimes 

 issue." (Levinsen, 1909.) 



"In Petralia chuakensis the perforated area in the dorsal surface is 

 for a broad radicle, which, however, is not always developed; a similar 

 area occurs in Petralia vultur, and also in Petralia japonica there is 

 often a similar large radicle. Levinsen refers to the radicle tube in 

 P. dorsiporosa." (Waters, 1913.) 



