80 TH. MORTENSEN, (SchwecL Südpolar-Exp. 
be taken into account any more in serious scientific discussions. — It may be ob- 
jected that the figure represents A. cordatus , while my observations are made on 
A. cavernosus, and that the differences might thus be due to specific differences. 
This objection does not hold good, for two reasons: i) it is extremely improbable 
that two so nearly related species as A. cordatus and cavernosus should prove so 
highly different in this early stage, and 2) I have examined a corresponding stage 
of A. cordatus and found it exactly agreeing with A. cavernosus. 
Pedicellariæ are developed already in this stage, both globiferous, rostrate and 
tridentate, the latter both 2- and 3-valved; they are all of the same structure as in 
the grown specimens; the rostrate pedicellariæ are only comparatively very large. 
The disk of the tubefeet of the frontal ambulacrum is small, the rosette plates are 
not yet developed. The tubefeet around the mouth show the first indication of the 
terminal prolongations. The spicules are few and small. 
The next stage, the youngest of the free- 
living specimens found in the material, is repre- 
sented by specimens of 4 mm. length. There is 
thus a small gap here. Meantime the develop- 
mental processes, which have been going on while 
the specimens have grown from 2.5 mm. to 4 mm. 
length, are easily enough understood. There is 
a complete series of specimens of the following 
sizes. In PI. IX are represented a series of figures 
of young specimens (4, 5, 5.5, 6, 6.5, 7, 8, 8.5, 9, 
12, 16 and 18 mm.) showing the changes under- 
gone by the test during growth. The tests have 
been cleaned and treated with alcohol-glycerine and 
show the limits of the plates upon the whole very 
clearly. It has, however, been thought necessary 
also to give some text-figures illustrating the minor 
features, which cannot be seen exactly in the phototypic plates, but which are ne- 
cessary for gaining a clear conception of the growth changes of the species. 
In the specimens of 4 mm. an important change is to be noticed, viz. that the 
transverse branch of the fasciole has been developed (Fig. 19), the whole of the 
fascioles of the species having thus been formed, viz. the peripetalous fasciole (anterior 
part of the primary fasciole + the transverse band) and the latero-anal fasciole (the 
posterior part of the primary fasciole). The transverse band is seen to pass between 
the apical system and the periproct, traversing the two pairs of interambulacral 
plates which have meantime developed between the periproct and the madreporite. 
As growth goes on this transverse fasciole becomes stronger, but it keeps its posi- 
Fig. 19. Abatus cavernosus. 4 mm. 
Showing the transverse band of the 
fasciole. 
