Part II. — Stichodactylince and Zoanthece. 193 



region, as is also the case with the two other species, rendering suitable sections 

 in this region difficult to prepare. As the sponge spicules are very long in P. inono- 

 stic/ms, much longer in fact than the thickness of the capitular wall, they are of 

 necessity disposed in a regular circular series, very obvious in thick transverse 

 sections (PI. xin., fig. 9). 



The outer part of the column- wall of P. Swiftii is loaded with inclusions, but 

 none extend beyond the encircling sinus. The mesoglcea there becomes extremely 

 homogeneous in structure, an included cell even occurring but rarely. Such a 

 strongly marked division of the mesoglcea into two parts — an outer, containing the 

 inclusions, canals, cell-islets, etc., and an inner, practically homogeneous in nature 

 and separated from the former by the encircling sinus — appears to be more or less 

 general throughout the genus. 



The size of the colonies and the extent to which the coenenchyme is developed 

 are likewise features of some importance. The simplest stage is exemplified by 

 P. separatus, where each polyp is distinct and surrounded by only the merest trace 

 of coenenchyme. To include this exceptional instance, I have slightly added to 

 the previous definitions of the genus. The few polyps in any colony of P. monosti- 

 chus also afford but a bare indication of connecting coenenchyme ; while in a colony 

 of P. tunicans or P. dichroicus, hundreds of polyps are, as it were, inserted in a 

 common incrusting coenenchyme. P. Swiftii is somewhat intermediate in the 

 dimensions attained by its colonies and the amount of coenenchyme produced. A 

 few polyps only constitute a distinct colony, each arising from a clearly separable, 

 though very limited, coenenchyme. 



The species I have examined, support the experience of Haddon and Shackle- 

 ton (1891, p. 623), that "all the members of a single colony of dioecious Zoanthese 

 belong to the same sex." All the numerous polyps of P. Swiftii and P. tunicans 

 sectionized were crowded with ova. It seems remarkable that of very many 

 examples of P. separatus and P. monostichus, microscopically examined, none showed 

 any trace of reproductive cells. 



Parazoanthus tunicans, n. sp. 



(PI. x., fig. 11 ; PI. xin., fig. 7; PL xv., figs. 4, 5.) 



Each colony consists of a thin coenenchyme from which numerous polyps arise 

 at short distances apart, the whole completely incrusting the main stems and 

 smaller branches of a large Plumularia. On the smaller branches the polyps are 

 arranged in a distichous manner, in a plane at right angles to that of the pinnulse 

 of the Hydroid, and the polyps on the two sides are either opposite or alternate. 

 On the thicker stems their distribution becomes more irregular, and the polyps 

 extend all round ; they often arise obliquely to the surface of the coenenchyme. 



