ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 
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flat epithelium ; (b) the internal connective-tissue layer with the spicules 
and their formative cells ; and (c) the porocytes scattered about more 
or less evenly in the wall. 
(3) The amcebocytes or amoeboid wandering cells met with in all 
parts of the sponge. 
The classification adopted is : — 
Class Calcarea. Sub-class Homocoela. Order Ascones. 
(1) Family Clathrinidae. Genera Claihrina and Ascandra. 
(2) Family Leucosoleniidm. Genera Leucosolenia and (?) Ascyssa. 
With regard to the origin of the triradiate systems, Minchin re- 
cognises a scheme of development common to all, with a number of 
constant and definite stages, as follows : — - 
(1) Formation of “trios” by immigration of cells from the flattened 
epithelium. 
(2) From the trios arise the “ sextets ” by division of each cell 
into two. 
(3) The spicule appears with each of its rays corresponding to two 
sister-cells of the sextet, i.e. to two cells which haye arisen from the 
division of one of the cells of the trio. 
(4) As the rays increase in length, the inner formative cells of the 
sextet remain at the apices, the outer formative cell at the bases, of 
the rays. 
f5) Disappearance of the apical formative cells. 
(6) The basal formative cells, after building up the rays to their 
full thickness at their bases, migrate slowly to the extreme tips of the 
rays, where they remain adherent as the definitive spicule-cell. 
The fourth or gastral ray is an adventitious element superadded to 
the triradiate system, and secreted by a mother-cell which is derived 
from a porocyte. The nucleus of the secreting cell may remain single, 
or may divide into two or into four nuclei ; but in all cases the cell itself 
remains undivided, forming a jflasmodium-like investment to the spicule, 
or at least to its terminal portion. 
In the theoretical part of his paper, Minchin discusses the origin 
and evolution of calcareous sponge spicules. Four theories are reviewed : 
— the Biocrystallisation theory of Haeckel, the Adaptation theory of 
Schulze, the Mechanical theory of Sollas, and the Alveolar theory of 
Dreyer. It is to Schulze’s view that Minchin inclines, namely, that 
the fundamental forms of the spicules are to be explained by adaptation 
to the primitive types of structure in sponges. 
He sums up as follows : — 
The first appearance of a calcareous spicule, both ancestrally and 
in development, was probably in a minute vacuole in a dermal cell, 
filled with an organic substance in which the minute sclerite appeared 
as a crystal or concretion. It assumed a non-crystalline form in adapta- 
tion to its secondary function of support, and the contents of the vacuole 
formed the spicule sheath. The ancestral form in Calcarea was a simple 
monaxon, placed tangentially, and completely imbedded in the body- 
wall, lying between two adjacent pores. 
From this ancestral form others arose : — ( a ) the primitive monaxon 
acquired a distal portion projecting from the surface; ( b ) groups of 
