22 PROCEEDINGS OF THE NATIONAL MUSEUM vol.70 



MICROPORELLA TRACTABILIS, new species 



Plate 3, Figures 3-5 



Description. — The zoarium incrusts shells and Cellepores. The 

 zooecia are distinct, little elongated, hexagonal ; the frontal is convex, 

 ol-namented with tremopores and with very fine granules. The aper- 

 ture is semielliptical, the peristome is thin, salient with straight 

 proximal border, ornamented with five or six large spines. The 

 ascopore is large, subtriangular, closed by a perforated lamella, sur- 

 rounded by a salient, often oblique peristome always adjacent to the 

 apertural peristome. On each side of the apertura there is a small 

 triangular avicularium with pivot, the beak oriented toward the top ; 

 the mandible is setiform and always long enough to touch the 

 avicularium of the adjacent superior zooecium. The ovicell is large, 

 globular, ornamented by tremopores like the frontal. 



Measurements. — 



[A«=0.06-0.08 mm. [Zs=0.55-0.60 mm. 



Aperturaj^^^Q 10_(3.i2 mm. Zooecium j/^=o.40-0.50 mm. 



Affinities. — The operculum is transverse and has a proximal border 

 somewhat convex, when the proximal border of the peristome is 

 rectilinear. The tremocyst has small pores. The ascopore is triangu- 

 lar, large, with a distal border somewhat concave ; when it is oblique 

 its form is that of a crescent. 



This species has absolutely the structure of Microporella calif ornica 

 Busk, 1854. Compared with the recent specimens collected at Santa 

 Monica, it differs from them in its avicularia placed higher above 

 the level of the ascopore, less oblique, smaller, in its neater frontal 

 verrucosities and in its larger zooecial dimensions (Z5!= 0.55-0.60 mm. 

 and not 0.40-0.50 mm.). 



Biology. — This is a very remarkable species because of its avicu- 

 laria. The mandibles are very delicate and rather long setae. Their 

 feeble hairlike dimensions compared with the great surface of the 

 cells do not permit us to consider them as organs of oxygenation. 

 Really they can scarcely agitate the surrounding water to an appre- 

 ciable and sufficient manner, especially under a pressure of 40 to 60 

 meters. But the mandibles are always long enough to touch the 

 pivot of the avicularia of the adjacent superior zooecia so that all 

 the avicularia of the same colony are in tactile direct communication. 

 Even on dried specimens this phenomenon is clearly apparent. The 

 avicularia are here organs of tactile sensibility, justifying the specific 

 name we have chosen. The simultaneity of the mandibular move- 

 ments is the consequence of this ingenious arrangement. These avic- 

 ularia are of little service to the zooecia which bear them, but they 

 assure the biologic unity of the colony. They give us a good example 

 of social discipline, each at the service of the other. 



