NURSING ECHINODERUS. 
57 
lacrum, and the primary tubercles of the latter bear two sets of 
spines of much larger size than are usually met with in such a 
position on the test of allied species, those of the outer circle 
being, however, considerably larger than those of the inner. 
These specially developed spines are cylindrical and nearly 
smooth, those of the rest of the surface being distinctly ribbed, 
and, during the breeding season at any rate, they are inclined 
towards the central point of the apex so as to form again a sort 
of open tent, into which the eggs pass directly from the minute 
ovarial apertures in the genital plates. 
In the only example of the other section of Echinoids the 
Petalosticha, or forms with petaloid ambulacral areas, which is 
cited by Professor Thomson, the principle on which the retention 
and protection of the eggs and young are effected is the same 
as in the preceding species, but the specialisation of the 
apparatus is very much greater. The specimens were dredged 
in January from depths of from 20 to 50 fathoms in Accessible 
Bay, Kerguelen’s Land, and the species is referred to Hemiaster 
of Desor, nearly related to the genus Brissopsis , which includes 
the well known Fiddle- Urchin ( B . lyrifera) of the northern 
British seas. 
In this species, which Professor Wyville Thomson describes 
at some length but does not name, the nurseries are four in 
number, and formed by a widening and depression of the paired 
ambulacra, which constitute four deep, thin-walled, oval cups, 
sinking into the cavity of the test. The ovarial apertures are 
as usual placed opposite the interradial areas, but by the mode 
of arrangement of the neighbouring spines a sort of covered 
passage leads from the opening into the marsupium, and along 
this the eggs pass. The eggs, which are very large, are 
arranged very regularly on the floor of the incubatory pouch, 
and each egg is held in its place by two or three short spines 
which bend over it (fig. 4). The occurrence of numerous speci- 
mens of this Urchin, with eggs and young in all stages, facili- 
tated the investigation of the process of development, and proved 
that this is effected directly. The eggs, when first placed in 
the pouches, are of a deep orange colour and enclosed within a 
soft vitelline membrane which they entirely fill. As the blasto- 
derm is formed they become paler, increase in size probably by 
the imbibition of water, and soon exhibit a whitish spot with a 
slightly raised border indicating an opening which is presumably 
the permanent mouth. The surface then acquires a translucent 
appearance, and becomes strongly tinged with dark purple and 
greenish pigment, when “ almost immediately, and without any 
definite intermediate steps, the outer wall is filled with calcified 
tissue, it becomes covered with fine spines and pedicellarise, a 
row of tentacular feet come into action round the mouth, the 
