NEW GENEVA AND SPECIES. 
Spong. 16 
break up into secondary rays. The ciliated chambers form what is 
essentially a single layer between the dermis and the basal lacunse, much 
as in Ilalisarca lohularis. P. monolopha id. ibid. pp. 407 & 448, pi. xx. 
figs. 1-7, pi. xxii. figs. 22-29, Trieste, Lesina, and Naples. Consists essen- 
tially of a sac with smooth proximal and deeply folded free sides ; ciliated 
chambers globular, opening directly, or by short canals, into the cloacal 
cavity ; the pores open by short branched tubes directly into the ciliated 
chambers; a ciliated pavement epithelium may be readily found on all 
surfaces except those of the ciliated chambers. The middle embryonic 
layer is slightly developed. The tri- and quadri-radiate spicules vary in 
the proportions of their different rays and angles. No central canal was 
observed in any of the spicules. Colonies hermaphrodite. The sper- 
matozoa occur in balls, closely resembling the ciliated chambers. The 
ova commence as large amoeboid cells in the gelatinous basis of the 
middle layer ; segmentation is dichotomous, and results in the 
formation of a hollow unilaminar blastula. After fixation two layers 
of cells appear, the outer a flattened epithelium, the inner con- 
sisting of irregularly polyhedric cells. A cleft lined with cylindrical cells 
then appears in the wall. The oscula originate at the edge of the crust, 
and are probably produced mechanically by the distension and conse- 
quent bursting through of the walls at one or more points. Young 
spicules are found each closely connected with a connective tissue cell, 
whose nucleus lies over its centre. P. dilopha, id. ibid. pp. 422 & 448, 
pi. XX. figs. 10 & 11, pi. xxii. fig. 30, Trieste. Has the connective tissue 
more developed subdermally and more compact than in P. monolopha. 
Cilia appear to be absent from the surface. The candelabroid spicula 
have two of their rays branched, and are distributed around the excretory 
canals as well as in the surface layer ; the histology agrees on the whole 
with that of P. monolopha. The larva show^ed a stage which appears to 
represent the formation of a gastrula. P. trilopha, id. ibid. pp. 427 & 
449, pi. xxi. figs. 1-3, Naples. Individuals hermaphrodite. The canal 
system is somewhat more complicated than in the other species, having 
subdermal cavities connected by canals with the pores ; at least three 
rays of the candelabroid spicula break up into branches. In all three 
species the spicules are variable, the variations taking chiefly the form 
of production of spines or rudimentary additional radii. 
Plakortis\^Plac-\ F. E. Schulze, (11) p. 449. Based on P. simplex, id. 
1. c. pp. 430 & 449. pi. xxi. figs. 14-16, Naples. Encrusting ; a distinct 
dermal layer, overlying a network of subdermal cavities ; no basal lamina 
or basal lacunar system ; excretory canal system arborescent ; a dense 
deposit of granular matter around the canal system and ciliated chambers. 
Spicules few, consisting of bi- aud tri-radiates only, chiefly lying parallel 
to surface, agreeing in general character with those of Plakina. The 
points of divergence from PlaJdna in the histology are but few ; the 
ciliated chambers are rather larger and fewer than in that genus. Prob- 
ably no cilia occur on the pavement epithelium. Certain large round 
inflated cells occur in some parts of the connective tissue layer. The 
biradiate spicula are never truly acerate, but always present some irregu- 
larity at the middle ; they lie chiefly parallel to the surface. 
