EEPOET ON THE TETEACTINELLIDA. 
217 
as a cover-cell, at least I imagine not, as the* slice in which it occurs is of extreme 
thinness and the section passes through the very middle of the spermatoblast cluster. 
In later stages also one observes spermatoblasts in which the outer wall has been 
reduced almost to invisibility, forming an outer layer to those less advanced which lie 
in the interior, and finally one can trace the filaments or tails of nearly mature sper- 
matozoa radiating outwards from the cluster, while the inner spermatoblasts remain 
in a state corresponding to that of the preceding stage. The nearly mature spermatozoa 
just mentioned consist of an oval head 0'002 mm. long, with the pale margin still sur- 
rounding the nucleus, and a tail which can be measured for 0’012 mm. in length. The 
excurrent canals in the neighbourhood of the sperm-bearing regions are partly filled 
with a finely granular stained material which consists of discharged spermatozoa. 
Skeleton . — Surrounding the strongyles one observes a thin layer of material of some- 
what higher refractive index than that of the sarcenchyma, and in places at irregular 
intervals this bulges out into oval nuclei, about O’OOG mm. in length, and containing 
a small spherical nucleolus. The nuclei are flattened against the spicule, and their 
appearance is suggestive of the existence of a layer of cells surrounding it. It is 
possible that an irregular film of spongin is associated with the spicules and that the 
nuclei are those of sponginoblasts. 
The strongyles vary considerably in the nature of their terminations, on the one hand 
pointing towards an oxeate origin, and on the other indicating a tendency to a more 
pronounced strongylate type. Thus in the average form a slight attenuation occurs as a 
preliminary to rounding off, and in many cases this becomes so marked that the spicule 
would be better described as an oxea with a rounded point ; in other cases, on the con- 
trary, the strongylation is abrupt and a typical strongyle results. In one or two cases 
tylostrongyles were observed, the terminal accumulation of silica which we may infer to 
have converted the oxea into a strongyle having proceeded a step further and rendered 
the strongyle tylote at one end. 
The strongyles are disposed in spicular fibres, which near the cortex are directed 
towards it at right angles, but away from it they appear to wander without rule, a general 
tendency to run parallel to the walls of the canals being, however, observable. The ortho- 
trisenes appear only at the cortical ends of the fibres, lying with their cladi extended in 
the fibrous layer of the cortex; hence their rarity in mountings of the separated spicules. 
The possibility of the formation of large asters by an overgrowth of small ones 
is suggested by the occasional occurrence of abnormally large globules (PI. XXVII. 
figs. 7-9), which sometimes occur singly, sometimes united together, two or three at a 
time. These show concentric rings of growth surrounding a central core of substance 
of the same character as the axial fibre of actinal spicules. The spherules may be fre- 
quently observed within the granular cells of the mesodermal sarcenchyma (PI. XXVII. 
figs. 16, 22). 
(ZOOL. CHALL. EXP. — PART LXIII. — 1887.) 
Err 28 
