202 



valve and with two opesiulse, while the kenozooecia, springing from the axial 

 zooecia, have a small depressed cryptocyst perforated by a pore. All septa have 

 a series of single-pored rosette-plates. Bivalve ocecia, in which each valve must 

 be considered a kenozooecium. No avicularia. 



Alysidium parasiticutn Busk. 

 Catalogue of Marine Polyzoa, Part I, Cheilostomata, pag. 14, PI. XIV, 



figs. 6—9. 

 (PI. VII, figs. 3 a-3 o). 

 The zooecia, which are rather elongated and trapeziformly rounded, steadily 

 increase in breadth towards the arched, distal margin. The aperture, the slightly 

 curved proximal margin of which is situated in the distal third of the zooecium, 

 is broader than long and has a glistening ridged distal margin, often with a 

 series of small tubercles. There is a membranous opercular valve and the oper- 

 cular arch is situated in the free margin itself. Almost the distal half of the 

 frontal surface is furnished with a depressed cryptocyst, which also occupies the 

 region between the aperture and the distal margin of the zooecium. The post- 

 oral cryptocyst, which stretches more than half-way back between the aperture 

 and the proximal margin of the zooecium and which has generally a number 

 (most often 10 — 15) of glistening tubercles, is separated from the remaining arched 

 part of the frontal surface by a semi-elliptical boundary ridge, which is very low 

 in the middle but increasing in height distally and ending on each side at 

 one of the horn-like spines, from which it is separated by a small notch. These 

 two spines, situated at the margin of the zooecium opposite the aperture and 

 standing out almost vertically from the surface of the zooecium, have generally 

 a form resembling that of short cow's hq|-ns but are a little more compressed. 

 In their proximal inner part each of them has a small hole, apparentljr leading 

 into the inner cavity. On the proximal side of the aperture on each side is a 

 rather small, irregularly rounded opesiula, the inner margin of which nearly 

 always terminates in a short, most often rod-like process, seldom with two or 

 several points. The two opesiulae are always of different size, but while in the 

 axial zooecia this difference is slight, it is large in the others where the opesiula 

 facing the axis of the colony is twice the size of the other. Immediately on the 

 proximal side of the two opesiulse is an oblique, glistening stria, which is how- 

 ever rather indistinct in the axial zooecia, passing right across the zooecium to 

 the lateral margins. This stria, which in zooecia of the second and third order 

 is inclined towards the central line of the colony, originates from a low ridge 

 on the inner surface of the cryptocyst, and immediately distally to it is the limit 



