354 E. A. MINCHIN. 
Fie. 69.—A gastral ray, of which the actinoblast is apparently giving rise 
to an excretory cell. 
Fie. 70.—A gastral ray, enveloped in an actinoblast with two nuclei, but 
without any excretory cells. 
Fie. 71.—A portion of the body-wall, seen from the gastral aspect, with 
the collar-cells brushed away; the figure shows, besides spicules and their 
scleroblasts and cells of the dermal epithelium, two pores (P.), a sextett 
(sert.), three ordinary ameebocytes (ame.), and numerous minute ameebocytes 
(amc'.). 
Fie. 71a, 71d.—Portions of sections showing the proximal ends of large 
monaxon spicules (I/.8P.) bearing each two formative cells (fc., ¢h.c.). In 
710 are seen also two cells of the dermal epithelium. 
Fic. 72.—Surface view of the body-wall seen from the gastral aspect; only 
the dermal epithelium (d.ep.) and the porocytes (p.c.) are drawn, to show the — 
way in which the epithelium lines shallow depressions, at the bottom of which 
the pores are found. 
PLATE 20. 
Figs. 73—100.—Development of the spicules of Leucosolenia 
variabilis, as seen in surface views of the body-wall. 
Figs. 73—88.—Development of the small monaxons. 
Fics. 73—75.—Formation of the two scleroblasts by division of a cell of 
the dermal epithelium. 
Fics. 76—78.—Youngest stages of the spicules; the lance-head embedded 
in the “thickener ;” the shaft, as yet scarcely formed, is being laid down by 
the “ founder.” 
Fries. 79—81.—The thickener leaves the fully formed lance-head and is 
travelling down the shaft, building it up to its full thickness; the founder 
prolongs the spicule at its proximal end. 
Fics. $2, 83.—Monaxons nearly full-grown, with the formative cells close 
together at the proximal end of the shaft. 
Fries. 84—86.—Three stages in the formation of medium-sized monaxons. 
Fig. 84 is a young stage, showing the lance-head still embedded in the 
thickener. Figs. 85 and 86 show spicules nearly full-grown, each with the 
formative cells close together at the proximal end of the shaft. 
Fies. 87—89.—Final stages in the formation of the large-sized monaxons. 
In 87 and 88 the two formative cells are still present on the proximal end of 
the shaft, which tapers evenly to a sharp point. Fig. 89 is a spicule com- 
pletely formed ; the proximal extremity tapers abruptly and bears no forma- 
tive cells. 
